I wanted to update with the results just in case anyone in the future finds this thread and wants to know what happened. I have been frustrated by some open-ended threads when I was researching.
There were many large changes made at once, so it is hard to say exactly what was most important. I built and mounted broadband absorption at the first reflections (although, as I found by reading many different threads over the years, there are many strong opinions about whether this is a good idea...) and bass traps in the front corners. I swapped in the new subwoofers, dual Rythmik L12's. And I moved the L/R speaker positions a bit further away from both the back and side walls, about 2 ft from each. It was probably about 10 ft to the MLP and the speakers were separated by 8-9 ft. I set up the two pairs of speakers in an AB AB configuration and switched between them using the Zone 2 on my AVR since it could level match.
The Emotiva T1s and the JBL 580s both sounded incredibly better under these circumstances. I can see now why both come highly recommended at their respective price points. I would say the 580 improved more than the T1 and had a clear edge. I'll be honest that it is hard to remember what the sub-bass used to sound like and if it is much different. It certainly sounds great and gets plenty loud and low. I actually think the L12s can go lower than my PB1000s could; there are parts of some electronic songs where a sinking bass note drops to 15 Hz and I literally felt the air vibrate with the L12s that were unnoticeable with the PB1000s. I am sure the PB1000 gets much louder in the 20-40 Hz range but it is not an issue with two subwoofers in my small/mid-size room. Overall, given how much better music sounds now, I would not be surprised if at least part of that is finding the servo subs to sound better.
Although I may have been happy just keeping the 580s, I had several other speakers on the way, so of course I had to try them out. The Ascend 170SEs didn't do as well, which is not surprising given their much lower price point. They were the very cheapest speakers I ordered, even before I increased my budget by a factor of two. I found the Revel M16, KEF LS50, Focal Aria 906 all to be fantastic. I preferred all three to the 580. They each sounded slightly different, but did not give an impression of being better or worse than the others. I personally slightly preferred the sound of the 906s. A few notes on the 906s: I have never really felt a good understanding of "soundstage" before, but these were incredible. The sound really did feel more enveloping than anything else I tried. They also had a much higher sensitivity, 89.5 vs the 86/85 of the Revel, KEF, resp. For those three reasons (slight preference for the sound, very "wide" sound, louder) I am keeping the 906s.
I ordered another pair of 906s to use one as a center. I will try to sell the other one... will take forever I'm sure, but I'm not in a hurry and a pair of 906s for $799 was only marginally more expensive than the CC900 at $699.
I will note that the LS50s seemed to do really well off axis, maybe because of the concentric drivers. Because I don't have or want Atmos and my surrounds (5.1 setup) are very close to the wall seats, they are raised about 2 ft from ear level (as I recall, this is what Dolby recommends anyway). Therefore I decided to get a pair of KEF Q150s for the surrounds since they were on sale for only $299/pr
. I have read they are similar in flavor to the LS50s, just a bit less detailed/refined. I expect they will have much better vertical off-axis than my current Emotiva B1s, which will be nice for the occasional 5.1 movie or concert.
Thanks to everyone who replied. Very happy with the new setup, the new speakers/subwoofers plus the room changes gave me the "wow" I was missing. I don't anticipate wanting to upgrade unless I have money burning a hole in my pocket several years from now.