I used to own for a very long time the SR-125, then upgrade to an RS-1 and then added an SR-325. After they all went bad (probably due to DC spikes) I got a SR-325is and finally his "Music-Pro".
I think over the years Grados have gone through quite some variations and although they all seem to have the original Grado "secret sauce" dating back to the days of Joseph Grado there have been inconsistencies to the point where some Sr-125's would outperform some RS-1's. In fact even a reviewer in an analysis in the Grado web page had indicated a problem with the over enthusiastic bass of the RS-1 bloating into the mids and highs a problem that was not present in the much punchier sr-125 kick drum umph.
Tom's specific Music-Pro is a surprisingly super fast super punchy Grado that easily outperforms most estats including Koss, Stax, and Jade that i own in the speed department let alone the midrange magic.