Right, just got back from another practice, a few things that hopefully might help you Ben.
I had no trouble balancing the MM/Crawler set today. I certainly got more output and a slight volume boost (I mean, really slight. In fact, it might just have been the fact it was a humbucker and maybe cutting through the mix better) when switching to the Crawler, but the set balanced perfectly well. In fact, I was getting quite a lot of power from the neck singlecoil when volume was wound up to full.
The Mother's Milk are very responsive to your attack. I had them set on volume 10 through the JCM800 (as before, both pre and master volumes were set around 6 or 7) and hitting them hard (as I generally tend to do) gave me plenty enough power to solo without engaging a fuzz pedal. PLaying softer, particularly on some arpeggio parts I was playing cleaned proceedings up somewhat, although there was obviously still some overdrive/distortion coming from the Marshall. So to get better clean tones, I'd roll the volume back to about 7 on the guitar. Rolling it back further almost seemed to cut the definition of the pup completely and so I seldom used this - although it might be a fault of the pots rather than the MM.
The best way I can think of describing the Crawler would be 'controlled agression'. It definitely has a tighter feel to it than my VHII's, and I feel it kicks my amp harder. Having said that, it can of course be controlled by the volume again and brought back to a more subtle, bluesy type tone.
Obviously, my VHIIs are in a mahogany/maple cap + ebony fretboard LP, and the MM/Crawler set is in an alder body, maple neck, rosewood board Strat, but I think the tone I get here is more powerful, and adaptable than the VHII set. However, the VHII + JCM800 gives you an absolutely killer crunch tone, which I don't get so well with the Crawler, it almost seems a touch more saturated. But I prefer soloing on my Crawler, whereas with just the VHII and an amp (no pedals) I felt a certain something was lacking, a little bit of 'ooomph' on the soloing note.
I'm sorry that I can't talk in technical terms, I'm not so good when referring to bass/mids/ etc; and I'm especially wary to recommend something in case it doesn't work out for the other guy - particularly when we play quite different music it seems - but I hope some of this can be some help to you!
Cheers, rambling over :D
Ian