I run them using a mac mini, Gustard USB to SPDIF converter > Adcom GDA 600 > Rotel RC 1070 preamp.
I am thinking of a new DAC
As long as your listening material is strictly 16/44.1 I would keep the GDA-600 myself.
That Adcom GDA-600 is a damn fine DAC,and IMO it'll be real hard to better it in the SQ department.
The GDA-600 has 2x PCM63 DACs in it,which are top notch multi-bit pieces.
That unit can easily go head2head against some VERY expensive modern DACs.
I'd try a different USB to SPDIF convertor before replacing the GDA-600,me I'd probably try something like the new Schiit Eitr.
And if I were still wanting to try out some other headphone amps,I'd be looking at dedicated headphone amps,and not a DAC/headamp combo.
As for the headphones,have you tried throwing some EQ at 'em yet ?
If yes what was the result ??
Me,I'm an AKG high impedance kinda guy (my #1 are AKG K240DF),so I needed ample power for those to sound their best.
I found what I was after with a NAD 106 preamp as it's headphone section can supply > 8V into 600ohms (that's a lot folks).
And my DAC is a Parasound D/AC-1000 which also uses 2x PCM63P-J DACs,so you can see why I said I'd keep that GDA-600.
Anyhow I guess what I'm getting at here is dont sell the GDA-600 till you're 100% sure you are gonna be happy with anything replacing it.
I'm just betting that beating/replacing the GDA-600 wont be as easy a task as you think it'll be,that's all.
Good luck here.
Bret P.