In case anyone gets here with the same problem...I believe I have found the actual cause of this problem. R9 is one of two resistors mounted over the trace that connects R49 to R11. In the amp I am currently repairing the end cap on R9 has degraded. The enamel that covers the the resistor has worn away and revealed the metal of the end cap. The lacquer covering the trace under the resistor had also worn and the two bare spots touched which bridged the node of R49, R11 & C24 and the node of C21, C22, R8 & R9 as referenced in BCD123's post above. This raised the voltage on C21 which caused it to overheat and "dome" the top of the cap (which is what I first noticed when I inspected the chassis...electrolytic caps should be flat on top!). The additional current drain is what caused R49 to overheat and burn out. Should you find either C21 domed or R49 burnt out check to see if this has happened to that amp. The glue that Marshall used to secure the caps to the PCB flowed around R9 which made it impossible to see that bridging impossible without removing R9 and doing some major cleaning! (The photo of the trace is shown after I repaired the trace.) Good luck to one and all.
I just clipped that lead of R9 on the board I still have laying around. I have the same thing on this one. Good eye. I'll definitely keep all of this in mind if I run into this problem again. Seems like an incredibly odd failure point, but by the looks of it, that trace could have been routed above R9/R10 to R11 which would have eliminated the issue all together.