Hi kevtn8,
Parts quality is all very good in the custom amp. If anything, I'd bet the transformers are better. Don't get me wrong, the 2A3 sounds VERY good. For example, for anyone who likes to listen to their Zen with the Bias on the hot setting, I'd bet they'd prefer the 2A3 for sure. It's just a matter of personal preference, and if I was stuck with the 2A3, I wouldn't be upset at all. In fact, I wish I preferred the 2A3 amp because the bass and extra power really is nice.
If I had to pick one, I'd stick to the Zen, but I'm keeping the 2A3 because eventually I'll probably end up with some Alnico's and I'd love to hear this amp with those drivers.
I use the stock tubes, including the SV83's. I've tried EL84's but prefer the SV's and yes, I'm using the 8ohm tap.