I have a XA30.5 and a X350.5 and have had two Jon Soderberg moded Threshold amps through out the years. Sonically speaking the Moded Thresholds sound very very good however the PASS sounds significantly better. I can tell you will need more power to drive those Magnepans 3.6 than a XA30.5. My advise is you should buy a used X250.5 or X350.5 if you can. They may not be 100% Class A but they will blow the doors off the Moded Thresholds. I had a pair of heavily moded SA/1 mono blocks with the Optical Biasing and balanced and a new right out of the box X250.5 not broken in killed the The Threshold Mono Blocks and by a huge margin.
XA 30.5 or mod S/500?
I've been watching the 'Gon fruitlessly for some time to get a Threshold S/500, "late Series II w/Optical Bias" (per Jon Soderberg, this is what he needs in order to do his modification to produce a Class A one-chassis SA/1 "clone" [my word, not his]). I'm beginning to wonder if a Pass XA 30.5 might not be equivalent or better sounding; I know they're easier to find. Unless I get lucky and find an already-Soderberg-ed S/500, the cost of amp and mod might even be approaching the price of a used XA 30.5(?). Either amp will be used with a FET 9 preamp. Speakers are currently Vandersteen 2CEs, but contemplating future purchase of 3A Sigs, or Maggie 3.6s, and a REL sub. Anyone heard SA/1s AND heard XA 30.5? Thanks.