You used honest to goodness Western Electric 300Bs... From their website, a matched pair is $1,500 dollars, and you have monos so times two... Well, good stuff doesn't come inexpensively. As for selling your Pass XA60.8s, don't do that; those are some very fine amplifiers. If you're using a Pass preamp, I bet that sounds pretty astounding. In my experience, Pass amps are "picky" when it comes to that "just right" sound. I went through a few preamps before I picked up a Pass and boy oh boy, I wish I had done that sooner. Nice job on your SETs.
First tubes in 30 years
I had tube preamps (CJ PV4, ARC SP6B, ARC SP9) in the 80's. They usually sounded great, but I had enough problems with them (tube failures, microphonics, background hiss) that eventually I switched to all solid state.
Over the last couple years, I've really gotten into DIY and decided it would be fun to try a tube project. I've been intrigued by the idea of a 300B SET amp, so I built a set of monoblocks. I started with the basic design (and transformers) from the ANK 300B Parallel SET C-Core Monoblocks, and added a lot of upgrades. Check out my virtual system for details.
I have to say, I'm really enjoying the sound. They are still in the early stages of breakin, with new WE 300Bs installed a week ago, but they are making a lot of music sound better than with any of my other amps (mostly Pass XA60.8s recently).
I'm planning to build a tube preamp as one of my next projects.