The two moderately priced tube models I auditioned for my HD600's were the MF X-Cans v.2 and ASL MG Head DT. In my opinion, the Musical Fidelity beat out the Antique Sound Labs for neutrality and dynamics, as well as bass firmness. It doesn't, however, sound particularly "tubey" (as opposed to the ASL), if that's what you're after. There were some aspects of the EL84-powered, transformer-output ASL that I slightly preferred in terms of spaciousness and liquidity (also jack and pot quality), but in the final analysis, it was just too colored and slow compared to the 6922-powered, output-transformerless MF (which was used alone - not with the X-PSU upgrade AC transformer). I'm a tube fan, but this ASL sounded excessively old-fashioned to me, while the MF was natural, articulate, and extended (and featured better casework). The MF adds a pair of loop outputs for signal pass-through as well, making it easier to integrate into systems where line-outs are at a premium for recorders, processors, etc. Guess which one I bought? As for the 'phones, I tried the Grado RS-2's, but found them bass-heavy, lacking a bit in clarity, and uncomfortable (just plain heavy) compared to the HD600's - whose prodigious resolution admittedly does come at the expense of a little thinness.
