Mp-1 Classic tube channels problem

[Davide]

Hi everyone,
this is my first post here :)
My MP-1 Classic has a strange problem: the S.S. channel works like a charm, the tube ones (clean, brown and dist) not so much.
The dist at least has got some good level of volume: but I've to put the OD1, OD2 and Master at high level and the sound is not so distorted, it seems to get only a crunchy level.
The brown channel is the one that virtually doesn't work: I've to put the OD1, OD2 and Master at very high level to hear a bad and confused sound, with a lot of noise.
The clean channel has got a very low level of sound if I set the master under 9 (and over) and if I don't set OD1 and OD2 to very high values (around 9-10) I can't hear anything than a hissy and weak sound.
I already replaced the R151 resistor (as suggested by MarshallJMP  ) but I didn't notice any improvement in the sound: yesterday I noticed a small burning on the tube board below the vactrol U22 and I was wondering if it can be the reason of the problem... I hope you will be able to suggest me a solution, you are my very last hope  8) Thank you guys. Davide