Hfe on those is max 300. At the 10mA running through these, the base current will likely drag the voltage too much. A darlington (2N6426 perhaps?) might work.

Could they? Sure. Should they? Probably not. You have neither sufficient voltage nor current in the SS application, the heater's need a lot of current and are too low voltage, the mu is too high, and they are tricky to work with.

Nice design :) I am a little concerned about the unbypassed cathode resistor - you are adding ~2500 Ohms to the plate resistance with it, which sort of defeats the point of the low Z tube. I have a write up of such designs which might prove useful...

Here is a quick rundown of what is upcoming ... DSHA2 is a small, direct coupled, transformer gain amplifier for use with balanced headphones. DSHA3 is kind of the default amp, meaning that if you ask me which one I recommend, this is usually the answer. It uses transformers for gain, and it...

There is an ecp thread? I didn't even know. Do you have a link? I'm not trying to ignore anyone, I just haven't seen it. I am a fan of the headphone power amp concept - basically, for people with a DAC that has a volume control, or for people who are going to just drop the amp into a rack...

There was a time when single ended amps were only used because they was cheaper to build and good enough for low end equipment.

Push-pull is a large catchall that is not very descriptive. Yes, it is push-pull, but this does not mean that it is very closely related to many other designs that can also be described as push-pull. It is a differential circuit, which is a better description here. The output impedance is very...

I may make it if Todd picks me up on the way.

A phono stage is probably the most difficult audio component to build. A tube phono stage is the most difficult variant. Here's one I built a while back, though there is not much more there than a schematic. Bottlehead makes several fine kits if that is more up your alley. Otherwise, pick up...

You mean grid stopper. Unlikely the issue unless it has always been there.

That sort of noise is consistent with something oscillating. If it just started recently and happens regardless of the tubes used, that suggests that a part has gone bad -- likely a capacitor somewhere.

If the amp is running as expected, they have around 150V across them. At startup, or if the tube is not conducting for some reason, they'll have 225V. And if the regulator fails, they could see close to 300V or so. If one fails, you still have the transformer between you and the high voltage...

Plastic is good. I think you'll be surprised at how good the box caps are. But if you aren't, the best option is likely a small (0.1u?) bypass across the top of the box cap. Otherwise, you might look for 4.7u/300V caps that will fit and use 8 of them.

Do those have metal bodies? Nope.

If you are investing this much effort, upgrade to the equivalent 300 series. You can remove the endbells and replace them with brackets. https://www.edcorusa.com/brackets