Something is going to give at some point. Every design is a compromise between response smoothness (or deliberate bass enhancement with any degree of care for the rest of the range, which will be its own compromise), sensitivity, materials, and price.
Even for an IEM having 117dB/1mW like the Westone 2 or 120dB/1mW like a Shure SE535 is relatively rare, let alone 125dB/1mW from Aurisonics now that it was absorbed by Fender and the ASG line killed off.
Sadly you're on a forum where people are willing to pay the sensitivity penalty with an amplifier if it gets smoother response (or just listen at a lower volume), so chances are apart from IEMs nobody might be familiar with anything close to the sensitivity of a mass market headphone whose primary design parameter is "make sure it gets loud on whatever people shopping at a big store are likely to have in their pockets." You're basically looking for a scooter among a bunch of people who have performance cars and if they ever use a bike as a daily driver it's something like an Aprilia Shiver.
Your best bet really is to get a PMX 60 again or whatever replaced it in Sennheiser's line so the sound will be close.
If the DT770 on what i assume were velour pads already too hot, the only way the M60X with leather pads can be any better is if you wear it forward and follow the contour of the earlobe...but that still leaves you hot wherever the earcup covers the rest of your ears.
Also if the DT770 at 96dB/1mW is too soft and the benchmark is a 122dB PMX-60 chances are you'll perceive a 102dB/1mW headphone closer to the first than the latter.
