Hi Pete,
A couple of observations from someone who has heard neither model -
1. The tube subs seem to have a reputation for being somewhat less musical than box subs. Likewise, I think the large size of the overall form factor might surprise you. The cylinder sub design philosophy seems to center around an enclosure that is efficient both in production as well as footprint. They go low, but people I trust (including chesseroo) have commented that they don't seem quite as fast as comparably priced boxes.
2. The EP500 and EP600 have VERY sophisticated digital amps that allow them to work collaboratively with the driver instead of those two processes being simply sequential.
3. Driver engineering still matters or SVS would not offer three different price points. Axiom is an R&D company, so I have no doubt that the EP series drivers do what they are supposed to do.
All that being said, my opinion remains that the current pricing on the STF-3 makes it too good to pass up. That would be a very noticable upgrade to your current sub, and either of the QS models would be a noticable upgrade to your current surrounds. I know I'm in the minority here, but I don't think everybody will get the same amount of value by spending significantly more just to avoid upgradeitis. Not everybody has M80 envy.
