Just picked up an rpm last night for a killer deal on craigslist.
Plugged it in today and messed with it for quite a few hours.
I found that for me it works best straight in front on my carvin bx500 rather than in the effects loop.
After setting my bx500 flat (added its graphic eq later to fine tune) and then started turning knobs on my rpm I found a killer gritty tone pretty quickly. And like someone mentioned before, I then just adjust the blend knob for each song depending on how much grit I want with everything else staying the same.
I am blown away at how good this thing sounds so far! It took me 3 years to find a great tone from my bx500

and in 30 minutes on this rpm I have a tone far far better
Saturday I will find out how it sits in the mix with a very loud guitarist and drummer. We play progressive drone type rock with lots of groovy mellow stuff too.
Here are my rpm settings for anyone interested:
DRIVE- dial pointing at the D in DRIVE (11 o'clock)
MID - full on boost to the right

MID SHIFT - 3rd line on left @ 200-250hz I guess)
BLEND - usually 2nd line left of noon (*adjust to taste*)
BASS - between noon and first line to left, points to A in BASS
TREBLE - 3rd line left of noon (10 o'clock)
I will report back after this weekend with any adjustments I make.
Thanks to everyone for this great forum full of awesome info!