Yes, that's it, I swapped the tube board but the mp1 is still sounding bad in tube mode, in ss mode sounds good.

To make sure, you swapped the tubeboard but the MP-1 is stil sounding bad?

Well, i swapped the tube boards today but still sounding like crap... I was hoping this to be the problem and now it is clear that it's not the case.

Q3 and Q4 are the 2sk364 type.

Any ideas of what else to look for? 

So that must be the elusive cause I've been trying to figure out for all these years.

I really love how the Microtube 100's sound when they are bridged.

Harley

You will only have this problem when running in bridge mode.

I think I better do that too - been running stereo lately

In respects to the MP-1 headphone output, in the manual, it tells you to connect a parametric EQ to the headphone out, and gives you EQ settings on it to emulate a mic'ed cab.

Harley

Yeah I do remember reading that too, but I can't find it in one of the manuals.

Ok thanks, it has been bugging me for days now, I really hope we can get it fixed 

Well you have 19Vac difference between the 2, that translates to about 26Vdc, add 5Vdc difference between the MP-1's (190Vdc vs 185Vdc) and you get a difference of 31Vdc. The transistors rarely fail unless you have a shorted cap. I have only seen it once.

Let's hope it doesn't take 4 months to arrive