Dual SVS PB-1000 Pro versus Dual Rythmik LV12F- does Servo Technology Matter.

With all due respect, I don't think" serious music listening and AVR' belong in the same sentence. For music I'd choose Rel. But for movie explosions et al I'd try the Rythmic

I think the servo tech in the rythmiks will decrease distortion allowing tighter bass that you want for music. You can't go wrong with Rythmiks for either application.

I am still mostly sold on the SVS but I wonder if the servo technology of the Rythmik might give me somewhat better performance for music.

I have to say I agree with @artemus_5 that debating between these two subs is small potatoes versus the negative affect the AVR is having on your 2-channel sound (although I do wholeheartedly agree with you idea of adding two subs as they’ll greatly improve both your HT and stereo experience). At this point I’d say get the SVSes as they’re probably cheaper and offer better integration tools, and use the $$$ saved to put toward the eventual purchase of a good stereo integrated amp you can seamlessly incorporate into your HT system with the front L/R pre outs on your Denon. You’ve got good speakers, and I think you’ll be amazed at how much more they’ll give you in 2-channel performance fed with a good stereo integrated. Hope this helps, and best of luck.

Using a home audio receiver isn't what I would consider "serious listening", but to answer your question, you won't go wrong either way. 

I have 4 subs in my main system, 2 are Rythmik, 2 are SVS.  They're both great subs.

I have 2 small systems in small rooms like yours.  One has a Rythmik F15 and the other an SVS SB-12 NSD.  If I had to choose, I'd go with Rythmik, but the SVS subs are very good and a little more bang for the buck. 

While I'm a proponent of multiple subs, you should be fine with a single sub in a room that small.  I'd go with a sealed sub.