Yes, with a medium sized room that is fully open to the kitchen, you actually have a large volume of air to try and move with your subwoofer(s). I have no doubt at all that your current subwoofer is your weakest link in terms of your gear. It is not surprising at all that things do not sound "full" or "balanced". You are quite literally running out of output in the midbass and bass.
As we've pretty much agreed, your current Primus speakers are pretty good. Yes, there are better speakers available for more money. But in terms of your current price point, the Infinity Primus speakers that you already have are among the best for that price class. So unless you simply want something "different" and not necessarily "better", I would suggest to you that a speaker purchase would not be the best use of your money right now. In the future, when you have significantly more money to spend, there are certainly better speakers to be had! But with a $600 budget, I honestly don't think you'd be fully satisfied if you spent that money on speakers. You wouldn't be getting much in the way of an upgrade over your Primus towers.
So if you're itching to buy a new piece of gear, I would recommend focusing on the subwoofer. You have a large space once you factor in the room being open to the kitchen, so I would recommend going for a sub that can genuinely handle a large room.
Some good candidates would be:
- SVSound PB or PC12-NSD (the PB is a large box while the PC version is a cylinder that costs a wee bit less and can be very handy as it takes up very little floor space by being tall. You can also lay the cylinder on its side, which makes it easy to hide behind a couch or along a wall)
- HSU VTF-3 MK4
- Outlaw Audio LFM-1 EX
Those three all end up being over $700 after you factor in shipping. The HSU VTF-3 MK4 is the most flexible of this bunch. It offers a lot of options for filtering and tuning the low bass output so that you can choose deeper extension or louder output. The SVSound PB or PC12-NSD is very linear and flat down to 20Hz, but it basically drops off a cliff below that. The Outlaw LFM-1 EX is essentially a clone of the older HSU VTF-3 MK3 subwoofer.
If you really want to keep the purchase around $600, you might start to test the output limits of the following less expensive subs. It will depend on how loud you like to crank your music and movies though. And the following subs should all still perform quite well in your room size. Certainly far better than your current sub!
- Rythmik FV12 - easily my favorite $500 sub. One really nice thing about this sub is that you cannot over drive or damage it. Rythmik has used excellent filters and limiters to protect this sub and it is a tight, clean, accurate sub that can almost hit right down to 20Hz. Personally, I don't think you can do better for the same price or less.
- HSU VTF-2 MK4 - a very close second to the Rythmik, IMO
- Outlaw Audio LFM-1 Plus - basically a clone of the older VTF-3 MK2. It's got a bit more amp power and you can choose to plug a port for lower extension or leave both ports open for higher extension with more output, so it's a very good option for larger spaces like yours.
That's by no means an exhaustive list of subs, but I figure it's easier not to overload you with options
These are all very good subs at excellent price points relative to their performance, so you basically cannot go "wrong" with any of them. They're pretty much all plain, rather large black boxes, but you might find one among them to have a certain feature or have certain box dimensions that make it a better fit for you. So look them over and see what you think. I can guarantee that every single one of them can absolutely crush your current sub
Anywho, from what you've said, I think a new subwoofer is likely going to make the biggest improvement for you. However, it also seems VERY likely that you have less than optimal acoustics in your room. Once you've got enough posts so that you can post some pictures, it sure would be a great help to have some photos to look at so that we can possibly make some suggestions for ways to improve your room acoustics and possibly improve some of your placement if we see something that can be adjusted.
Hope that helps!