I own both amps. I have had the Pass XA30.5 for about 4 years and have been very happy with it. It's performance has been excellent driving my Focal 1028Be speakers via an ARC LS27 preamp, DAC8, PH6 and WTL Versalex turntable.
However, the ARC Ref 75 is simply a more soulful sounding amp. It offers a bigger deeper soundstage, richer tone, great dynamics, natural detail, and a generally more relaxed and realistic presentation. Some things tubes just do better than anything else. And remember, I like the Pass and have enjoyed my time with it. But the Ref 75 has that special Magic. In general the Ref 75 reviews I have read online were accurate and said basically the same thing.
However, the ARC Ref 75 is simply a more soulful sounding amp. It offers a bigger deeper soundstage, richer tone, great dynamics, natural detail, and a generally more relaxed and realistic presentation. Some things tubes just do better than anything else. And remember, I like the Pass and have enjoyed my time with it. But the Ref 75 has that special Magic. In general the Ref 75 reviews I have read online were accurate and said basically the same thing.