Hmm...tricky one this
I have not heard the REL 505 for myself. I can only make an educated guess based on its design and reading through the reviews as to what sort of performance range it might be in. It's a moderately expensive 12" sealed design with a 500 Watt amp. There's no magic. That sort of design is only capable of so much, no matter how much it costs.
Given the way you've typed out SVS PB12+, I'm guessing you've got one of the older versions that used the old BASH amp?
I think what might really help would be if you could describe what you like best and what you would like to see improved with both of your subs. I'm familiar with the PB12+. I would expect, just based on the design, that the PB12+ can play lower and louder than the REL505, but that maybe the REL505 sounds "tighter" for music - it just can't reach as low as loud?
If I'm on the right track with that guess, then it would really help to know what you are hoping to get for your $2500 budget. Are you generally happy with the PB12+ performance in that particular room, but you just want louder output with less distortion? Would you rather have the tighter sound of the REL 505 in there, but the REL 505 just isn't up to the task of playing loud enough? Maybe you're after the best of both worlds?
We just need to know a little more about what you're after.
You've said that a second PB12+ is honestly probably too large for your current setup. So I'm guessing space IS an issue? How about a tall sub that doesn't take up much floor space? Would one of SVSound's cylinder subs be a viable option?
I can tell you ahead of time that one of SVSound's "Ultra" series subwoofers would be a significant step up over either of your current subs. The box version PB13-Ultra DSP at $2000 delivered or the cylinder PC13-Ultra DSP at $1700 delivers a solid 3dB more output across the board vs the current "Sledge" DSP version of the PB12-Plus, which itself delivers a solid 3dB or more across the board than the old BASH amp'd version of the PB12+. So a current "Sledge" amp DSP Ultra will deliver a solid 6dB or more across the board than your PB12+, and that's with vanishingly low distortion - considerably better than the PB12+. The only question is whether a single subwoofer that's about twice as loud and has noteably lower distortion will satisfy all of your desires. If you're looking to improve the frequency response across multiple seats, a lone subwoofer is restricted by placement and room interaction and EQ and filtering can only do so much. If you're mostly just after more output though, I think an SVSound "Ultra" subwoofer would be a fantastic choice. The cylinder especially, if you don't mind the looks, costs less and takes up a lot less square footage.
Going from your PB12+ to a sealed sub - or a pair of sealed subs - could be quite the change in overall bass sound in your room. A lot is going to depend on how much room gain you get on the low end - and that will also depend on specifically where you place the subs. Just straight up, you're not going to get the same loudness of output in the really deep sub 30Hz range from a sealed sub vs your PB12+. So if it's all about getting more output with this purchase, a second REL 505 or a new pair of sealed subs might not be the best choice for this room and this purpose.
Anywho, some stuff to think about and some more info to provide and questions to answer
I think a new SVSound "Ultra" will likely provide what you're after, but it's going to depend on several other factors. So get back to us and hopefully we can figure out the very best choice for your upgrade
