I am having this very exact issue with my Mesa MK III
Seems to mostly only happen if something is in the loop
(tried many different effects and cords to rule out)
Will have to try the contact cleaner thing.
Really?
People generally aren't boosting thier amps to add a bunch more gain.
They add a boost to compress and tighten up the amp, and to make it more responsive.
The DSL's stock are somewhat "muddy" sounding under higher gain.
Adding a boost in front of the amp resloves this and brings...
I would try and get the head if you can for around $500.
I would pass on the cabs and pick up a used 1960 4x12 for $350 or so.
Much better sound quality having one good cab over 2 crappy ones.
I would not do that trade.
If your using the normal input on the RR, I don't really see the point of getting the 76'?
The RR should already be able to pretty much nail those tones, plus you also have the RR mode with more gain.
In addition, the 1959RR's will hold and rise in value for...
Red Channel OD 1 Gain 5-7
Treble and presence at "3"
Mids 5-7
Bass 5
Use a tube screamer/Bad Monkey type of Boost in front with Gain on pedal set very low or off, level 5 and up.
Add an EQ pedal in the loop for further punch.
Greenbacks will smooth out even more.
JJ EL 34L's...