I'm mainly working out of a small home studio.
Theres a friend who runs a commercial studio and I know he has a couople of MPX-1 in there, I should ask him what they are like
What appeals to be about the MPX1 is all the routing options, for eg inserts into the feedback path of the delay is *very* useful.
Im my world all delays should have feedback inserts along with both pre and post eq/damping - it would be written into law
Kind of thing I like doing is high EQ + distortion + phaser in a delay feedback path as a for eg...
I couldnt tell if the 550 provided that kind of flexibility.