I have Blutarsky's Duet here and I am not sure it sounds any different than the HP out of my mini-DAC. The Headphone out of the Duet or mini-DAC is pretty good with stock D7000 already, which we tried at the 11/1 CO head-fi meet along side my modded D2000 that sounded almost the same, but with slightly better treble extension due to my APS cable. The Duet HP out is pretty good and on par with other $250-400 headphone amp only boxes.
One portable that I know is great with the modded D2000 is the Meier 3MOVE which helps fill in the mids which are a little more recessed than those on the D5000, and the D7000 mids were just like those on the modded D2000. But, the 3MOVE and Duet may still be close to the same level, except the 3MOVE is just a little smoother in the highs than HP out of my Apogee mini-DAC or the Duet.
But normally I would recommend a desktop amp with lot of current output if you are trying to upgrade from the Duet HP out. The minimum amps I have found to be just a slight upgrade over the Duet would be something like Grahm Slee NOVO or Head-direct EF1 or Travagans Red with AD743 opamp upgrade (or OPA627), or Headroom Micro Amp. But if you are looking for a more noticeable upgrade you'd have to spend a lot more than those $250-400 options. In my house, the next jump up in sound quality is my upgraded Single Power Square Wave XL, which would cost $1249 if Mikhail could get his act together and actually ship stuff. The modded D2000 run balanced out of the Single Power is a nice upgrade over the Apogees, and you don't need a balanced DAC to get the sonic benefits from the balanced drive with this amp. Since Mikhail is so bad about delivering orders, I would recommend the Headroom Balanced Desktop, and re-terminating your cable to balanced (and get a single ended adapter). It will also convert the single ended input to balanced, and the Duet DAC is likely every bit as good as the optional one for the balanced desktop, and 98% as good as the Apogee mini-DAC.
The D7000 were also pretty nice out of a stock Woo WA6 set for low impedance phones at the 11/1/08 meet, and even better out of my maxed WA6 with PDPS mod and Sophia Princess rectifier that took it to the same $1250 as the Single Power.
I have never heard the Mustang, or other intermediate amps like KICAS Caliente, or LD Mk VI. But they may be good options too. Another great amp with the Denon (at least my modded D2000 but not tried D7000 with it) was the Headamp Gilmore Reference Amp which was extremely transparent and well controlled with Denon when I had the amp on loan last month or two, and a used one might turn up for between $600-800 if you look. I would still prefer the WA6 over the Gilmore Reference because of how it sounds better with my RS-1 and HD600.
So, if you are not going to spend 1.5 - 2x as much as the Duet costs for your amp, you might not find the improvement worth what you spend.
!
