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