best amps new and used for Infinity RSIIb speakers

I own Infinity RSIIa. Almost the exact same 'cept you have slightly better woofers, stronger EMIM magnets. I use a Forte' 4a 50 watt per channel amp and it is fine. Rewiring the speakers with Audioquest Midnight wire helped a lot. Did that rewiring 7 or 8 years ago. A tube amp IS the way to go BUT you have to be prepared for the extra expenses a tube amp will entail. There ARE solid state amps that can be used, like my Forte'. Some of the new Bryston amps might be good. If today I had to replace my amp, Bryston is what I would look at first.
Also the little box (that comes with the speakers) to go between the pre and amp (for bass control) can be helped by putting in really good RCA jacks. The stock ones a super-cheap junk. I used Kardas Silver RCA jacks.

I owned a pair of RS2Bs for several years. You definitely need to use tubes. CJ or ARC would be ideal -- even some of their older stuff. I also owned a Music Reference RM9 II and that was very good as well. The more power the better but 100 or 125wpc will do fine. Good luck.

ARC VT-100 (I have the Mk. 2) is magical with the RS-1bs: liquid, huge, airy soundstage. They were originally designed with Conrad Johnson tube gear, BTW.