I would agree with the last comment. I have had the same Pro Lambda's and the accompaning "box" for over a decade now. Whereas they are incredibly clean and revealing (at least with my passive pre and forte 1a power amp) they are a bit hard as you say and I believe this is due to the transformer box. The way to go is to get the stax class A tube amp made for these headphones and connect your pre directly to the stax tube amp. Having your stax headphones going through a McInstosh receiver could only make them 'harder' sounding.
Stax Lambda Pro Headphones--Can sound Hard??
I bought a used pair of Lambda "Pro" phones with SRM 7 box. They sound very good and generally better than every dynamic phones I have tried, but also sound a bit "hard" in the high frequencies,. I only occasionally use them, so I don't want to sink any big bucks into "mods" or expensive interconnects. I have them wired to a McIntosh 1700 receiver speaker outputs to avoid hooking the transformer box to my amp. I also have a cable running (Audioquest "Coral") from my preamp to listen to CD, tape and Turntable. Any suggestions on improving the overall sound, and eliminating the "hardness" Thanks, Jim