The reason you don't get the punch form the MMGs is that they just can move enough air at the lower frequencies. That is the "loudness" issue I mentioned earlier. It's also the reason that you will be better off with a higher crossover point, say 80Hz. That way your sub does more work and you will get more punch. I owned MMGs for awhile and try the various settings. 80Hz is your best bet, IMO.
I've experimented with it quite a bit, and I think what you say is true -- particularly if I'm looking for extra punch in rock and roll type music.

However, most of my music collection doesn't require the extra punch, being either: 1)music that is already mastered with a fair amount of bass, such that the hump would be really obnoxious; or 2) the style of music that just doesn't need it.

So for now, I think my solution will be to simply turn up the crossover freq on those occasions when I want to rock out, and at all other times I'm leaving it around 55-60.

Unfortunately the Musical Fedelity 3.2, (while it has "pre-out" connectors), doesn't have "main in" connectors. This makes it basically impossible to send a high-pass-only signal to the main amp. There is no handy solution to this problem that I can think of.
Yeah, I figured that out at some point. It's OK though, I think I'm quite happy with things now - I'm done fiddling and I'm just going to sit back and listen for a while. (Although I reserve the right to change my mind about that!)