I'm hoping you'll take what I write in the spirit in which it's intended: I'm here to try and help! And I genuinely want for everyone to get as much performance for their money as possible!
Polk's subwoofers are...well...they're not very good
Sorry, but that's the easiest way to say it. For the price that you would pay in a store for a Polk subwoofer, they just don't play as low, or as cleanly, or as accurately as what is available for a similar price.
Now, that said, getting a second, matching sub to the one you already have for FREE. Well, nothing can compete with that price!
Selling things and then trying to decide on a replacement/upgrade can be quite the hassle though. My honest advice would be to take your friend's free PSW505 for now and really work out the absolute best PAIR of positions in your room in order to get the most even and accurate bass across as many of your seats as possible in your room.
I am ALL for using multiple subwoofers. But the mere presence of more than one subwoofer does not guarantee better performance. You can't just take two or four or eight subwoofers and plunk them willy-nilly and expect improvement! Every room is unique and you basically either have to just go through lots and lots of trial and error with positions, or you have to measure carefully with accurate measurement equipment to help you determine where it is best to place your pair (or more) of subwoofers. The idea is to get your multiple subwoofers to work TOGETHER to create more even, flat and accurate bass at multiple seating positions in your theater. If you mostly watch alone and really only care about one, "throne" seat, then it isn't nearly as important to use multiple subwoofers to create that more even and flat response in other locations.
What you're basically doing when you use more than one subwoofer is you're creating a "phantom" subwoofer. It's similar to what happens when you listen in just stereo using only your two Front L/R speakers. Using just two speakers, you can still create a "phantom" center image, where it seems to your ears that the sound is coming from directly between the two Front speakers, rather than from two separate speakers on either side.
So with multiple subwoofers, you're creating a "phantom" subwoofer in between the actual subwoofers. What this allows you to do is to place that "phantom" subwoofer in a location in your room where it wouldn't be physically possible to place an actual, physical subwoofer (ie. somewhere in the middle of your room, not along a wall or near a corner, which is where it is usually most convenient and feasible to place an actual, physical subwoofer).
So one thing that you can do is to "crawl for bass". What you do is put one subwoofer in the spot where you normally sit (push your seat out of the way
). Now you play something with a lot of continuous bass. I like to use the THX Optimizer bass sweep, which you can find on any THX certified DVD or Blu-ray disc. The bass sweep is the last chapter in the "Audio" section of the THX Optimizer menu, so just set that chapter on repeat and let the bass sweep play over and over!
Now what you do is crouch down so that you are at seated height. We call it "crawling for bass", but it's more like "duck walking for bass"
Normally, you'd "duck walk" along the walls and near the corners where you are most likely to place an actual physical subwoofer. But if you are using multiple subwoofers, you can "duck walk" anywhere in the room! What you are looking for is the spot where that bass sweep sounds (or better yet, measures) as flat and linear as possible all the way from the highest note in the sweep to the lowest. You will find, as you "duck walk" around your room, that there will be spots where the sweep sounds like a roller coaster! It will get MUCH louder and MUCH quieter as the sweep plays at certain notes. These peaks and nulls are the nodes of the standing waves in your room. Every room will have standing waves, there's no escaping them completely! But you should be able to find a spot somewhere in your room where the sweep plays all the way from the top to the bottom without any big changes in volume.
Once you find that spot, mark it! If you were only using one subwoofer, you'd want to put your subwoofer on that spot. Again, this is why you'd normally only "duck walk" along your walls and near the corners where it would be convenient and feasible to actually put a physical subwoofer there! But if you have the freedom of multiple subwoofers, you might mark a spot somewhere in the middle of your room. Now it's a matter of placing your two (or more) actual physical subwoofers so that they create a "phantom" subwoofer over that marked spot!
There's a bit more to it than it just being in the middle between two subwoofers. The reason there's more to it is because bass waves reflect really well off of walls, ceilings and floors, so it isn't exactly the same as that "phantom center" in a stereo image. With strong bass, you also have to factor in the sound that is reflecting off of the walls, ceiling and floor. That's one main reason why in a perfectly rectangular room, the best locations for a pair or quad subwoofers is right in the middle of each wall - or at the mid-way points of two opposing walls (ie. the front and back wall, or the two side walls). In a room that isn't a perfect rectangle though, it's more a matter of trial and error. That said, once you've "duck walked" and found that best position, you'll know what you're trying to achieve, and it's just a matter of fiddling with the placement until you get it!
The fact is though, many people do not have the freedom to place multiple subwoofers in the best positions to get that "phantom" subwoofer in the perfect spot! Most of the time, people only have a few places where it's feasible to place a sub. For that reason, multiple subwoofers are not ALWAYS better than a lone subwoofer. Sometimes, if you're really unlucky, you'll wind up with a bunch of cancellations or giant peaks created by the pair of subs interacting, which is worse at your prime seat than what a lone subwoofer would create!
Bottom line: multiple subs CAN be better than a lone sub, but they are not ALWAYS better, regardless. You have to be able to position them correctly in order to reap the benefits! But there ARE benefits to be had.
Getting back to your situation, I'd recommend that you take that second, FREE PSW505 and see what sort of results you can get with a pair of subwoofers in your room. This is a very inexpensive way to learn a lot about how bass works and to find out what works well in your particular room! I wouldn't pass up that opportunity to learn for basically no cost!
Eventually though, I'd recommend that you upgrade those Polk subwoofers. I don't know how much money you could sell them for, so I don't know what sort of replacement you'd be looking at. If you can scrape together $500-$600 for a new subwoofer, you can certainly get a MUCH better subwoofer with something like the Rythmik FV12 (my personal favorite for a $500 + shipping sub), or an alternative like an HSU VTF-2 MK4 or an Outlaw LFM-1 Plus, which are very good $550-ish subs.
Below that $500-ish price range, you're only going to find subs that have definite compromises. That is NOT to say they are bad. Far from it! It's just to say that, below $500, you have to decide if you're willing to give up the very lowest bass below 30Hz (something you are already doing with your Polk PSW505 sub) in order to get better accuracy, or if you're willing to give up "tightness", distortion and accuracy in order to get deeper bass with more "rumble" for movies, but not the best cleanliness for music. Once you get to $500 though, and something like the Rythmik FV12, you can pretty much "have it all" so to speak, with good, deep extension AND good accuracy.
And, of course, if you can scrape together around $1100-$1200, you can get a pair of these $500-ish price range subs
Above that price point, there are certainly improvements to be had for more money. But I feel very strongly that a $500 sub like the Rythmik FV12 gets you firmly, "through the door" into very good bass with very few compromises. From there on up, it's just a matter of getting more output, even lower distortion, even slightly "tighter"-sounding bass, etc. But it's a matter of degrees, not a huge, entirely new ballpark.
I hope this long post will be helpful
