Recheck the 6L6 pin no's. Pin 7 is one side of the heaters, I wouldn't expect to see a wire from the B+ running to this pin, nor diodes.
There should be a wire that feeds pin 4 from the B+ rail (screen supply), via 470ohm resistors (yellow/violet/brown, 1W rating or better), one for each tube. I'm not familiar with this model so can't say whether the 470ohm resistors are mounted on the tube socket or circuit board. When you find them, make sure that they measure 470ohms and neither are blown (a blown screen grid resistor can cause weak output). If one of these resistors is blown, make sure that you also replace & rebias the power tubes as it's most likely that one of these casued the resistor to blow in the first place. In fact, I would assume that you are using good known tubes?
A voltage check at 6L6 pins 3, 4 & 5 and plate current would be useful too (then just out of curiosity check voltages at pins 1 & 6 of the PI tube, if a PI plate resistor burns up you get low volume & filthy tone).
The diodes you mention sound like spike protection (don't appear on the schematic) and would more likely run from pin 3 to pin 8 (ground). If so, you could temporarily clip them out (it's not essential but a good idea to have them) to eliminate them from your enquires, replace with the same type & polarity when you're sure all is OK.
Can you be more specific about the voltage drop on the filter caps? The schematic shows that the screen supply has 2 filter caps in series at the screen supply node. Therefore, I would expect one cap to have twice the voltage on its positive end (connects directly to B+ rail) than the "+" of the other cap at that same node (this point connects to the "-" end of the first cap and the junction of 2 smoothing resistors, each resistor bypasses a filter cap).
You say volume is weak? Is it weak & clean or weak & dirty?
Confirm those pin no's before going any further.