Marshall JCM800 4010 - explain these circuit tweaks

[Tone Meister]

Just picked up a Marshall 4010. Amp sounds great, no issues whatsoever, very quiet. All electrolytic caps have been changed to good quality parts. Had some scratchy pots, so I opened it up to clean those and check it out.

Amp has much more preamp gain than I would expect from this circuit. Not a bad thing at all, just an observation. When I had the amp open I noticed a few non-stock parts and wondered what effect they might have on the amp.

Here is what I observed:

R8 - stock 10K/1W - changed to 22K
R26 - stock 47K - another 47K tacked on top to swap over from 6550 to EL34 power tubes
R22 - stock 100K was replaced with another 100K
C18 - stock 47p was removed completely [edit]

What effect will these minor changes have on this circuit?

Schematic attached and a few photos that might help.

[Richard]

My suspicions:
R8 - the higher value will drop plate voltage on V1a and V1b. This may increase distortion.
R26 - this will lower the range of bias voltage (make it less negative)
R22 - Negative feedback circuit. this is a pretty low stress job, I don't know why this would be replaced unless the value was changed and changed back to stock
C18 - I see a 47p on the schematic, I believe that this is to tame some high end by mixing the out of phase signals of the phase inverter. There is what looks like a .022n that may be C16, that a coupling capacitor but I don't think that the amp would work properly without it.

[Tone Meister]

Good catch on C18 Richard. I edited the original post to reflect the correct value.