doesn't look like much, but here's my "mod" on my th900. shortened the cable by a lot and put on an oyaide 1/4" plug...the other half is now an xlr->1/4" adapter.
I think you need a PM-1. ('great phone by the way; I have it and the TH900).
I use the Schiit Gungnir/Mojo stack and in spite of the forward nature of the amp, I've not found treble to be piercing generally. But then again, I have one track (by ELO) that I find to be pretty sibilant when played through any of the DAC/Amps that I own. So track quality matters. The TH 900 is no exception here. The mitigating factor for me with the TH 900 is that I do not have to raise the volume past 9 pm.
Hollowness might come from the staging?
As a matter of interest, what other headphones do you have with which you don't notice this hollowness?
If anything, I experience this more with my Stax Lambdas than I do with the TH900s.
Yes, you're asking for too much. FWIW, I had and sold the HE-6...the LCD-2/3/X/XC were all preferred by my ears (and more comfortable too).