Regarding adequate bass: I have a single REL S-812 to support the two Sopras, and if necessary could add a second S812 for stereo subs, but i'm staring with 1. I hope to have bass needs covered with the single REL.
Regarding adequate power at 30w:
I talked to a few Luxman dealers yesterday to get their take on the adequacy of power to drive the Sopra 3s in a big room. In short, it sounds like the 30w refers to the Class A power only, but the amp generates more like 100W per channel. None of the dealers had any concern at all that the 590AX would be adequate, which made some sense to me if this is their flagship integrated. I called Luxman in NY directly, and was blown away at how rude and unhelpful the guy was that answered. Even though the number I called at Luxman was for "Sales", he refused to answer even a basic question regarding power, which is fine if that's not his role, but the rudeness was a real turn off when trying to spend 10K on an amp.
If anyone is interested in better understanding the power of the 590AXii, I found this lab analysis and write up. Page 3 has the "Lab Report" info on the power which was in line with what the dealers were saying: