Okay, retried the Pro700 MKII for the second time, this time seriously at my friend's house in the silence of his room.
Now, I finally heard the "HOLYCRAP" bass that everyone was raving about... seriously, if the M50 is a muscle car, the Pro700 is a monster truck 0.0
The bass (the selling point of the Pro700) is very boomy, with significantly more impact than the M50 as well having a larger quantity as well (say, a 70% increase?). The general sound is warmer than the M50 though, something I did not like very much.
Then, I put on the M50 pads. The bass went from "HOLYCRAP" to absolute diarrhea-inducing madness. The quantity instantly came up to XB500 standards but was definitely cleaner-sounding than the Sonys. However, the bass did creep up to the mids slightly on some bass-heavy parts of certain music and impact was reduced a tad bit, a problem which the Pro700 did not have with stock pads on.
Versus the HFI580, surprisingly the HFI580 has EQUAL bass impact as the Pro700 with stock pads on. The HFI580 is significantly cleaner sounding with more forward mids and better treble extension. The Pro700 obviously wins in bass extension. However, I felt that the HFI580s have BETTER detail retrieval than the Pro700, but then again the HFI580 are very bang-for-buck...
Are the Pro700s worth the upgrade from the M50s? I say, not really. You're paying an extra 50 bucks for a warmer sound and more bass. Both provide the same level of detail pickup, but the M50 sounds cleaner and more lively. The Pro700 sound darker and warmer, which to me is a little too warm considering that the M50s are already very warm to begin with.
Unless you're looking for these traits (warmness and massive bass), I don't think that the Pro700 will provide the $50 worth of upgrades from the M50. Personally I would go for the XB500.
Then again, if you're a newbie looking for a bassier M50, this will get my recommendation.