I posted this a few weeks ago and still hasn't been solved. My amp was working fine and one day when I was playing loudly, the volume suddenly dropped.
The distortion is very quiet and fuzzy and the clean channel is so quiet that I can hear the normal guitar sound over the amp volume even when I am on 10/10 volume. The volume decrease is permanent and the distortion is very fuzzy.
My amp is a Marshall DSL 201.
I have replaced and biased the power valves and also replaced the preamp valves but it didn't solve the problem (although the amp is very slightly louder). When I play through the FX loop, the volume is even lower than through the normal input jack. It is nothing to do with my pedals or my guitar or my pickups or my cable or the input jack because the hum coming from the amp is usually very loud and buzzy but I can barely hear it even on 10/10 volume.
I noticed the power transformer didn't get hot, only a bit warm however the amp was only on for a few mins when I tried it after replacing the valves so it might not have got time to heat up (the valves were very hot however).
Any ideas on how to fix this? Thanks
get a new set of tubes, sounds like the power tubes are shot. JJ EL84 are your best bet to keep costs down. Remember new is just a word, I have gotten many new items including tubes that were shot. Did you bias the amp or did someone else?

if the tubes are good, then something else is messed up and taking it to a good tech would be my advice.
2002 PRS CE22
2013 G&L ASAT Deluxe
2009 Epiphone G-400 (SH-4)
Marshall JCM2000 DSL100
Krank 1980 Jr 20watt
Krank Rev 4x12 (eminence V12)
GFS Greenie/Digitech Bad Monkey
Morley Bad Horsie 2
MXR Smart Gate
Last edited by Robbgnarly at May 17, 2016,
Assuming you have tested with a different cord and guitar...

1 Test ohms across your speaker and confirm that it reads as expected +/- 10%.
2 Find a tube test rig and confirm all tubes are within specs.
3 Download a schematic and test values at each point from input to speaker output.

If you dont have the tools or talent to handle this basic process of elimination it may be time to visit a qualified amp tech. Fiddling blindly insid a tube amp can kill you quick.

Good luck!
"Your sound is in your hands as much as anything. It's the way you pick, and the way you hold the guitar, more than it is the amp or the guitar you use." -- Stevie Ray Vaughan

"Anybody can play. The note is only 20 percent. The attitude of the motherfucker who plays it is 80 percent." -- Miles Davis

Guthrie on tone: https://www.youtube.com/watch?v=zmohdG9lLqY
If you haven't tried changing the power tubes, I'd do that.

Check the impedance's of all your individual speakers, I recently killed the output transformer in my Ampeg VT-22 due to a blown speaker. Is there any bad smell from the amp when it was in operation?
..I was watching my death.