Your comments are correct Hellzero, especially about the "Q" control. Fortunately the Yamaha PB1 has all three, sweep, gain & Q, and is perfect for room ambience corrections. The slope on the Noll is not at the extremes either way
However my main question is what, if any, does anyone think the audible difference is between applying PEQ at the initial signal (instrument) or at the preamp stage of the amp.
Of course the other tone settings at either end of the chain will have a big influence, but I am assuming "flat" in both cases.
Ultimately the only thing to do is to try it, but I thought a bit of exploratory enquiry might throw up things I have not thought about, or indeed others experience.