I haven't done the direct comparison of ST-85 to MB-125, but I have compared the MB-125 to the ST-150 (which retail for about the same price).
Based on your stated desire to increase midrange transparency and dynamics, my suggestion would be to focus on the ST-150. The 6550's in the ST-150 have a more transparent, dynamic sound than the EL34's in the MB-125's, not to mention the extra output power.
If you get a chance to audition both (or all 3) on the same speakers/source (preferably your own) it would be best...but that's not always an option.