I don't necessarily hear any boosted treble in the M91ED, although the bass and mids are rather plush. It sounds pretty nicely balanced overall to me.
After digging the M91ED, I recently bought a so-called "better quality" V15 Type IV, thinking it would be heavenly. I was sorely disappointed, and resold it after a few months of trying to love it. The M91ED is still king on my vintage tables (AR/XA and Dual 1019).
Boosted/plush...well let's say there's "plenty" of nice deep bass and lovely crisp treble.

About the M91 and V15 type 1V...The M91 series likes capacitance load of 400-500pf BUT the V15 type 1V NEEDS 200-300pf only.So this would suggest your phono leads capacitance value is suited better to the M91.
So no surprise the V15 type 1V sounded crap on your system.
Also the Type 1V has the much flatter response similar to the M95 etc which is a very different sound to the warmer sounding curve of the M91.
In the right arm with the right capacitance load and good stylus the type 1V can sound really ace but it's a more "palpable", more revealing delivery than typical M91 & M75 design delivers.
However if you ever get the desire to try another cart you could go for a V15 type 111 which likes the same cap' range as the M91, being in the 400-500pf range.
I have a "Sassy" V15 type 111; same in a type 1V and my first "Sassy Shure" which is the venerable M91. Have to say that IMHO there ARE noticeable differences but they are pretty subtle. The SAS gives the M91 the hugely increased trackability, sonic performance, detail rendition etc that most of us believed we had to spend much more to achieve.It really brings the M91/M75 performance to within a bee's doodle of that of the V series cart's.
The AUD/US $150.00 approx for a SAS is serious bucks BUT look at the cost of high grade stylii for say Ortofon, Grado, etc and the SAS is cheaper than some of those. But I regard my Jico Sasses (sasses??) as the most immediately obvious audio upgrades that $150 bucks have ever yielded....and NO I don't have any commercial interests with Jico Japan ! lol
Just happy to spend the time hoping to help other AK members enjoy da music.
Cheers guys...