I do think the stock tube is the problem but you need to get a NOS tube new tubes can be shrilly in the highs, the electro-harmonix while I found to sound very good did have some metallic highs, magni 3 also has a forward sounding upper frequency range but its still smooth sounding and a warm amp, The brightness might be from you dac, but the MCTH does have a clarity advantage when compared to the other amps mentioned it was much clearer sounding than my lyr with expensive tubes, by clarity I don't mean more highs it was actually smoother sounding as well, it was just more clear, open and spacious sounding with overall better detail and depth to the sound.
Maybe you should try giving it a burn in or something.
I think shrill is a good word. And yea I do think that it does have a bit of a clarity advantage, the one thing I noticed is it’s a lot more airy and the soundstage is bigger too, really opens that up. But it has a sibilance to my ears, it can be fatiguing to listen to.
As far as the tube how long does it take to burn in? I’m the second owner so it’s not like the tube is brand new. I’ve heard this amp leans toward bright, so whether or not it’s worth trying a new tube I’m not sure, if anyone has any suggestions for warmer ones that don’t cost half the price of the amp that would be good.