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.
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.