Maximising headroom

[sub]

I think dynosoar1 has right in volume drop...
I have a PPIMV in my amp (1Mohm resistors, 1Mohm dual pot), and when i set the pot about 2 o'clock (i measured it was about 220Kohm) the amp volume significantly dropped.

Well, yeah, if you turn down the volume, the volume goes down...
The power stage hasn't changed though. It's still capable of 18 watts if you drive it fully.

I didn't touch the volume knob...

[leslie]

However, another frequent observation made about Marshall 18W style amps is that they start to break up at around 3 or 4 on the volume knob and after that point, don't get a lot louder, just get dirtier and more compressed.

This might be true if you playing humbucker equiped guitar.
But with single coils you get significant difference in loudness beetween 4-8.
Also with single coil you get more headroom,so if somebody wants more headroom there you go...