I seem to enjoy tormenting myself. Did some more listening tests this evening. What I found out was that some of what I was hearing WAS due to the amp (LD MkIII), if it is an impedance thing or just the general design I don't know. I found my SR325is had much better bass and were much less treble happy on my Headroom SS amp than on the LD, to the point I didn't really like listening to some songs with the Grado's on the LD MkIII. I did notice that I could listen to a song using my Senns and it would sound good on both amps, but different. The Headroom seems to have a mid-bass boost to it, which makes the HD600's sound almost boomy, but really helps the Grados. The real question is...and always is...which is correct. Does the Headroom have a particularly warm sound to it, or is it neutral and the HD600's have a bit of mid-bass boost. Are the Grado's bass shy or is the MKIII a little lacking in the bass area. The really funny thing, that had me chuckling to myself is that tube amps are supposed to be the warm sounding amp and in this case the SS amp sounds warmer than the tube amp. Just goes to show that any amp can sound pretty much how the designer wants it to, just a matter of what you like.
Just to end this on a Grado positive note, first time through I was listening to the Grado 325's on the Headroom amp. Got so lost in the music the entire cd played through before I remembered to try the Sennheisers <LOL> I will say the more I compare and listen the more I really want some RS1i phones. Seems to me that might be close to my ideal phone. Little more bass and a little smoother treble than the 325's but still keeping the Grado sound.
I honestly believe it is a compatibility issue. The most simple OTL amp will make a famously hard to drive headphone sing (Bottlehead > HD800) but not be best for your Grados. Likewise your Headroom is not optimized for your HD600s. The Grados are really efficient so they sound ok in some ways with some OTL, but not really. It's not that any amp is bad or voiced a certain way. Some amps can drive both. I had a Mapletree Super 2 that was great with every dynamic I could throw at. Other SS amps with gain/impedance switches can also drive both well, but not all amps can and OTL/Grados is not the best fit.