The difference in "slam" you are hearing is that the M15 has an "over the ear headphone" mode - or "high current" mode that delivers that slam. You aren't going to get that same M15 "slam" sound out of the A90 because the M15's difference is in it's onboard power / bass boosted headphone amplifier - it is not from the M15 DAC, so changing your DAC won't find the M15 "over the ear headphone" mode sound.
I have both the M15 and A90 as well, and really enjoy both - the A90 / D90 MQA more so now that I have gotten rid of most of the noise problems between the A90 / D90 MQA / PC with
cabling changes.
For me the cable changes I made dropped the noise floor so much I am hearing things in the sound stage from right to left without gaps and the sound has become very much more revealing. The bass "slam" has improved too, and I would say I'm much happier with the A90 / D90 MQA sound now than I was before the re-cabling.
Now I wouldn't say that I like the M15 sound better overall than the A90 / D90 MQA, but before the re-cabling it did sound that way to me with some headphones on some material. It's not that the A90 / D90 MQA sounds bad, it's that I enjoyed listening to the M15 sound - and on some headphones and material I preferred it.
On the road to trying to get that M15 "slam" out of the A90 / D90 MQA - I've found I can vary the sound characteristics of the A90 a bit by changing the Gain from L, to M, to H by progressively reducing the drive power from the D90 - using D90 PRE mode (variable output) instead of D90 DAC mode (fixed at 100% output). I vary the D90 output from -3.0dB to -30.0dB, and depending on the material the A90 output can be thinner or thicker and adjusted to taste. I also like to use the D90 remote control for volume adjustment.
My thought was that running the A90 in H Gain with less drive from the source - to match the dB output - may help the A90 gain some more "slam", I'm still playing with this endeavor. It was not enough to get the full effect of the M15 "slam", but that tuning was all done before yesterday's re-cabling, so I'm going to have to re-visit this tuning later.
@burma - I hope that detail helps.
I haven't tried feeding the audio output of the M15 into the A90, so I can't say whether we can capture some elements of that M15 "slam" through to the A90. Daisy chaining multiple AMP's, powering up the M15 when it could be resting, doesn't appeal to me personally. And, I am even more happy now with the A90 / D90 MQA sound after the re-cabling, so for me I am happy to enjoy both the M15 and A90 / D90 MQA systems separately.
Please let us know how you work out improving the A90 sound to more closely match the M15 sound, I'd like to know what cabling you use - maybe I'll give it a try - you never know it might be nice.