I can't directly compare what you asked (the ST-500 vs. the mAMP), but I have the mAMPs as well as a pair of SX-1000s. Here was my comparison between the two I made a couple of months ago that I sent to the Wyred folks:
"I noticed that somewhere between the 125-150 hour mark, the bass on the mAMPs really came together (am now wondering if it was also the STP/SE breaking in). I must say that now, the mAMPs are fantastic. I think the bass is pretty similar to the SX-1000s (perhaps a little more tuneful), but it is in the mids and highs the real changes are evident. The mids are definitely more 'dense' and seem to have a smoother presentation -- vocals sound more natural. The mAMP seems to alleviate the hint of dryness of the 1000s (never really noticed it with the 1000s till the mAMPs came into the picture). The highs are a dramatic improvement. There is much greater air and detail, but yet the presentation seems to be less shrill. Again, never really noticed this with the 1000s till I increased the volume of the system to extreme levels (I like to do this ever so often). But now, with the mAMPs, the 1000s actually sound a little hard and dry in the highs in comparison."
I used to own the ST-500, and at the time, I thought the SX-1000s were a decent step up, so I would think the mAMPs would be a noticeable step up from the ST-500. One thing to mention is the STP/SE really seems to mate well with the mAMPs. I was running a DAC2 directly into the amps, but definitely liked the STP/SE in the mix. I do have extremely efficient speakers (Zu), so that may also be part of the issue - the DAC2 was never able to get out of the 20s in volume level.
