Quick question , a while back I asked about some good amps for the Grado and several people suggested the LD Mk1+ hybrid amp. What I was wondering is, would an all tube amp (I am thinking about the LD MKIII) be as good or is there something in the mix of tube and solid state that really 'works' for the Grados. I just recently read a review of the Mk3 and it sounded like a tremendous amp. In addition to my Grado I will also be using it for my Sennhieser HD580's (think HD600). I know the mixture of a low impedance and high impedance phone makes the choice a little more difficult, or at least that is my impression.
So last year when I put together a desktop system, I wanted to try the Valhalla, even with 32 ohm Grados as my #1 cans. It's an OTL amp (my other amps all used output transformers) with an output impedance of 25 ohms. My experience mirrors the comments you already recieved -- no problems, not fat, not shrill. This combo is transparent, beautifully balanced, organic (but not slow or "old school" tube soft). In fact, it's fast and incisive when the music is.
If you haven't before, try "all tube" and see how you like it. What's more fun than throwing our hard earned $ away on this stuff?