Tom Schlangen said:

But finding a suitable relay capable of reliable
switching the B+ rail (and all that stored energy in
the filter caps) at, say, 350V-500V and some hundred
mAs continous current running seems to be no easy
task for the electronics parts shop around the corner...

For those applications I use a heavy duty 12V loudspeaker relay with
both contact sections in series, and a snubber cap across both.
Each contact section is specified as 250V max. (Omron).
Several amps using that circuit are running for more than 8 years now,
so this method apparently works.

--
Sander deWaal
"SOA of a KT88? Sufficient."