1) No, the Senn Hd 600 are not muffled and lack sparkle IF you use the right amp. The problem is that they are really hard to drive. Its really only a couple of amps really good for it, Lehmann/Matrix is one of them.. Other amps make senn sound closed inn, small soundstage, really muffled, and scales really badly. But a really good amp really transforms them, but it needs a really powerful amp.
2) I have dual ad797 in my matrix-m stage, I dont think they are better than anything else, they are so balanced that the vocals disapear. There is no magic in the vocal spectre, the mids are just "shrunk". I dont understand how people cal this music.
I think the burson is the best... i had the burson headamp before, really raelly wide soundstage. Lots of detail, super smooth..
So which headphones are efficient enough for the Forte????? !! AKG K 701 maybe? They sound good on a lot more amps than senns.
Kristian