I'll assume we're still full-on Class A. This 20W minimum still puts us in the moderate 845 B+ territory. Not exactly a bad thing.

I have been itching for a serious experimenter's power TX. Multiple taps on the primary like Varian requested from Paeco. Change the primary hook up to yeild 500-800V after an L-C filter Full-Wave with vacuum or Hg-vapour dioides. 300 mA deliverable at even the highest voltage. Probably ~$250 each. Slightly less if several are ordered.

On the 813 thing, I have not yet heard any, just an assumption I can get them to work well for me. The PP 845 is complex enough, and I am sure of that one. And I have OPTx's nearly tailor made for it. This is not a small thing to consider, availability of reasonably priced excellent OPTx.

As a swing for the fence( with a paid-off pitcher ), the PP845 amp is quite appetizing. 30W ought to be available fairly conservatively, and with no exotic parts. If it weren't for the 813's challenges, I'd build one next.
regards,
Douglas