Andy. That sounds like a temptation too great for Keith to resist:-)
Can you tell us more about DHT preamp valves, please.

DHT small tubes were made in the 20s and 30s in large numbers, and some
after that until all production died out eventually. All that is
available today is NOS (with one or two boutique exceptions - may be
useful for high end), unlike DHT large tubes which are still in
production (300b etc). Apart from availability they have two problems
which need to be overcome:
a) They can be microphonic, from airborne/chassis induced vibrations.
Users need to find a solution for this, which includes mass in the
chassis, decoupling feet, O rings, differential stages instead of SE,
or any combination.
b) Although they run off good DC like a bench supply, to sound their
best they need a sophisticated filament supply including a current
source.

When you have dealt with the above you are rewarded with a clarity and
delicacy which gets quite addictive - you can't go back to directly
heated small tubes, judging by the colleagues I have that use them. You
then start to stockpile the little buggers since they are shooting up
in price. I have 500 so I'm pretty future proof, and that's about the
numbers my friends have stockpiled too. I'm counting on the fact that
they're an investment - hope I'm correct. They're certainly getting
more expensive month by month almost. Dealers still have some, but it's
usually ebay.

Many are familiar with sticking a DHT at the end of the amplification
chain, but put it right at the start in the preamp and it makes even
more of a difference. Andy