Just checking, are you using the balanced output? It will give you a bit more volume compared to the single ended output.

With a pair of Sennheiser HD800s I find the volume at 11 o'clock is quite loud and 12 o'clock is obnoxiously loud. I don't have a pair of Abyss Diana TC's but do have a pair of Abyss Diana V2's which are only slightly more efficient, on them 12 o'clock is quite loud and 2 o'clock is obnoxiously...

I use the iFi iDSD Diablo as a portable solution with the Stellia. When comparing this to my desktop solid state amps the biggest difference I notice in the sound is slightly less bass punch on the Diablo.

Here is a photo of what came with mine. I know nothing about tubes, but based on them having "USA 5963" printed on them and mostly rubbed off red/orange ink I think they are RCA 5963.

You are more optimistic than me! Glad to hear they treated you well. Thanks for sharing your experience, I was more cautious about what I plugged into my A90 Discrete for the first few days as a result.

That’s better than I was expecting, given they explicitly state they will accept no liability for damage caused by a failure of the amp in the manual.