This is exactly my experience with the m15. For me after 40+ hours it opened up and really started to perform on reference level. I only use full size cans And its perfect. Worth every penny!

Exactly my experience too

I liked the M11Pro, but the forward mids was a deal breaker. How does the M15 compare against the M11Pro in mids tuning?

Balanced.

So the newest state of the art DAC chip sound like a R2R? Must be very very poorly implemented.

Denafrips is not ordinary R2R, they finally broke very long line of poor R2R DAC implementations on the market.
Their R2R performs well by any standard, also in objective measured performance against delta-sigma (dS).
Even entry level R2R Ares II from Denafrips delivers very respective measured performance - https://www.audiosciencereview.com/...s/denafrips-ares-ii-usb-r2r-dac-review.11166/

In other words, Denafrips measures like high end delta sigma, while providing immediate transient response (lifelike dynamics, tempo), much better multitone measurements which translates into exceptional music instrument separation, unreachable for any dS DAC, and no out of band noise related to dS design.

Somehow Denafrips fixed R2R THD+N, IMD problems and avoided all compromises related to dS tech in their products, delivering superb measurements and subjective sound quality. Last points applicable to FiiO M15, this is what I meant by comparison.

I thought the M11 pro's mids were too forward. i also thought the overall tonality had a type of 'harshness' to it: It sounded like the amp was trying to hard, difficult to explain.

Agree with AudioBomb, M15 is more balanced comparing to M11 Pro.
M15 has flat sound, maybe little warm. With M15 you should have no issues with any kind of harshness or graininess.