Here is a quote by the doc himself. Unlike me this guy knows his internals. This is a post in that link I posted.
"Really, if you tear them apart and look, the Xenon is actually the best built PG amp so far. All the parts fit perfect on the Xenon, and nothing is really a “one time use” fastener.
The Xenon also uses top quality parts as far as transistors, capacitors, resistors, and others are concerned.
Xenon won’t rust like a ZX, ZPA, MS, M, or Ti will.
Xenon actually weighs a lot considering it is all aluminum. You get more heatsink with a Xenon than you do with any other PG except for maybe the MS. My guess is the Xenon actually has more heatsink than an MS does.
I hate to keep defending the Xenon, especially since I don’t own one, and have no really plans to, but they really are great amps."
This guy has tested many many amps, and he knows his stuff. With that being said I would still rather have the tantrum due to size issues. The xenon is such a large amp. 22 inches long I believe Despite the early bacth issues the xenons are actually the most reliable pg amps to date.