As promised, I'm reporting back.
New receiver and subwoofer have been set up, ran the XT32 optimizer a couple of times, got it all adjusted nicely. The sound is very good and a noticeable improvement from before.
It is not excellent, however. The sound feels a little muffled to me. Also, I expected that the much better subwoofer would allow my MS speakers to focus on the higher frequencies and therefore sound crisper, but it doesn't feel like that's the case. Any ideas as to why these things might be happening?
Also, I wonder if what I did for the subwoofer cable is ok. I'm using my old subwoofer cable, which has one male in (now plugged into subwoofer 1 out of my AVR) and two males out (one now plugged into the LFE input of the Rythmik sub and one unplugged). Is this arrangement ok, or do I need a new cable given the Y-split in mine?
Finally, I'm a little disappointed at the lack of native support for Spotify in the Denon. I can stream it from my phone and that works well, but I can't play it directly from the AVR like I can do for Pandora - am I missing something?