Actually, I was not referring to how loud the headphones could play. I meant at loud listenable levels (and I occasionally listen at very loud volumes), I check for whether the high frequencies maintain their balance with the rest of the frequencies. And that's more of a check on the synergy of the whole rig and source media than just the headphones.
But I've found that amping tends to flesh out and boost the lower frequencies, which in turn tends to attenuate the higher frequencies (as compared to not amping). And I usually find that I prefer the effects of amping.
Take for example the RX900. Even after felt modding and at high volumes, its highs remained under control. That was from a XS71 sound card, no amp. Again, same was true for the JBL 410 Reference. I suspect the highs in these headphones are quite different than that of an MB Quart or the GMP 8.35.
Based on my experiences while here at Head-Fi and FWIR, the GMP 8.35 seems to be headphones that are more sensitive to synergy and preference than most. And, I don't suspect it would be to my liking with my usual equipment and source media: TV // internet radio + XS71 sound card // FM radio + Lo-Fi receiver.