I've not heard the Belles, not sure many have, but the XA30.5 is the best amp I have ever heard and imagine the 60 and 100 sound very much like it. In my mind, you just can't go wrong with Pass for sound, build quality, and support. If you did not feel the need to stick with Class A, the Ayre gear is what I would compare to the Pass.
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Weebeesdad, I think it is just a matter of taste, and to some extent the speaker you are using. My speakers don't need much current or damping, the shine and sound their best with tubes. If I were using Thiel 3.7s I'm almost positive I would prefer the Pass to any of the tube amps I've tried, with the possible exception of the CAT JL2s which can take control of almost any speaker. Have no doubt, that in the world of SS, Pass is among the best you can buy. I think the current crop of Pass amps have a 15kohm input impedance, and I would think you would be ok with any tube pre with 1000ohm or less output impedance, the lower the better. It is a hard decision, but ultimately I chose tubes over the Pass, but I think my speakers had a lot to do with that decision.
According to the Pass Labs website, the XA-.5 series amplifiers have input impedance specs of 30K ohms balanced and 20K ohms unbalanced. The May 2009 Stereophile review of the XA-30.5 states an unbalanced input impedance of 15K ohms. I don't know where the discrepancy in the unbalanced input spec originates, but John Atkinson's test measurements showed an unbalanced input impedance of 20K ohms.
My XA30.5 defintely said it was 15kohm unbalanced, maybe the monoblocks have higher impedance? I would ask Pass Labs. If it is 20kohm, slighly better yet for tube pres, but I do think you use many tube preamps even with the 15kohm rating if that is correct. I talked about this with Pass Labs before buying the XA30.5 as I knew I would be using a tube preamp, and I remember feeling comfortable that I would not have any audible problem with my preamp at the time, the Joule LA-150 MKII.