Subwoofer Amp Suggestions?

If you want to consider plate amps, I really like Rythmik Audio. You can have your XLR's, Class AB operation with 370wpc, designed for 4 Ohm drivers. Brian, the owner, is a great guy to deal with, he's been around a long time, and his products are very reasonably priced.

Per OP::
'Class D not my thing.' So NO Hypex for you!

Personally, I love 'd' amps but.........
OP limits suggestions to basicall A or A/B amps.

What does OP use to drive remainder of system? Some people would say that makes a difference and the sonic character of what drives the sub amp makes a large diff.

I didn't ask before, but does can each driver have its own amp or do you need one single mondo? If a single amp, than one of the huge Bryston monos makes a certain amount of sense as would Pass ($$$) or even a Mac. OR how about one of the MF 500watt 'superchargers'?

Magfan thanks for your thoughts. See the system - hybrid tube NP-220 for the mid/tweets and tube-like Aleph 2s, which are probably not ideal given current limitations, from 60 to 220 Hz for Maggie bass panels. This system integrates perfectly on the 3.6s but the subs need some deep extension. My thoughts ranged from a Pass 250.5, Modwright 150 SE (a little expensive used but appears similar sound), to a Classe CA-2300 (very low damping factor and very high current drive). Bryston might be ok but....

I have tried: 1) a used rebuilt 250 watt/channel Threshold S/550 which was not very bass extended, 2) an Adcom 5802 (bad luck - blew up),3) an Adcom 5800 (amazing killer bass extension but fan noise drove me nuts and now a little old), 3) a Counterpoint SA-20 (sonic character matches the rest of the system but only modest bass capabilities).

System is amazing. I can "turn the bass up" with the active XO volume controls but I need something with extension down to DC.