experience with Threshold SA/2e or S/500 amps?

Don't get too worried about the S 500, look for the S500II Stasis version. It's Class A to about 50 watts and then turns to Class A/B.

Nice amp, accurate, what you put in is what you put out.
I sent my Muse 160 II amp that had been doing sub duty for me for some time back to Muse. In it's place I had S300 II stasis, and put it in, not as big a difference as going from a B&K 442 to the Muse, but it was there, my subs integrate a bit better with the maggies which is no small chore.

Some people will laugh but, a sub amp makes as much difference as a driver change. Even a non audiophool buddy heard it one night.

He's an old friend and spending the night with his wife, so us phool were up late consuming adult beverages and as
we were feeling no pain, and I being in an X-perimental mood, swapped out the B&K for the Muse between sides of MSFL I-Robot, wholly smoke, he asked what happened. The B&K ain't no slouch or lacking for current drive, but it sure made a HUGE DIFFERENCE. The T-hold was even better.

loon

I use a slightly modified S/500 Series II. I would describe it as very dynamic, open and neutral, but, not necssarily warm. The S/series are class A/AB, the first 20% of their power running in class A into an 8 Ohm rating. The pure class A SA Thresholds are reputed to be warmer and more recesssed. I'm confident that any of the Threshold's with enough power for your room and listening levels can "control" your speakers. The SA and Series II amps are reputed to be more stable than the ealier versions. The factory e series provides for true balanced (upgraded versions have XLR in-puts, but aren't truly balanced) twin trannies, and are DC coupled. The e series are said to have tighter bass, but, the earlier single ended amps are said to have a sweeter top end.
I'm not familiar with your speakers, other than they are not particularly easy to drive with their 4 Ohm, 85dB rating. The S series will double down, but as they do so you may expect their level of class A opeation to halve. If you prioritise warmth; go with the class A SA/ amps. If you prioritise power; go with the higher powered class A/AB S/ series. The higher the power of the S/ series the more base class A you'll get. Perhaps the S/450e might be worth consideration. The older the amps, the more likely they may be need of a tune up with perhaps some new caps. Jon Soderberg of Vintage Amp Repair can be relied on to do this correctly.

I have used many amps to run my Infinity Renaissance 90's both class A and A/AB. Currently I have a Threshold S500 II as my primary amp for these speakers. Prior to aquiring this I was using a Threshold 400A 100w/ch class A.

Infinity's have a reputation for providing a challenging load and mine are no exception dipping to 2ohms with the watkins woofer. The S500 has added significant depth and control to the bass with only a minor compromise in the 'Air' and space of the sounds image. This though is likely due to the fact that I have not yet updated the capacitors. I believe the Stasis 2 is the same design as the S500 only biased to run in class A mode. Likely minor power supply differences. If you like the imagining and detail of the class A better, the Stasis 2 can be upgraded at the power capacitors, I know of one that now runs 240,000uF with 4x60,000uF capacitors from the original 4x29,000uF. This will beef up the bass tremendously and I am likely to do the same with my S500 as this two can be installed.

The current capability of the Stasis is unbelievable and should not have issue with your speakers when updated.

check the thread at DIY http://www.diyaudio.com/forums/showthread.php?s=&threadid=94114&highlight=stasis+3
for a discussion about this.

Funny that this thread popped up again. I been recently thinking about it. Perhaps, the extended frequency range caused me to dismiss the warmth aspect of the S/500 Series II. In retrospect it does have a touch of warmth. Regarding the proper load, most Threshold's are recommended for loads above 4 Ohms. That said, they are tested into loads below that and probably shouldn't have much difficulty with lower impedances.

My former loudspeakers had an impedance of 1,9 Ohm at some 87 Hz. The Threshold didn't have any problem with it, even when playing loud. It also drives my current Apogees with ease, so I wouldn't worry about the load.