They're actually really good, with (I believe) Wolfson converters in them. But not quite sure what you're trying to achieve? Are you after more volume, more clarity etc? That one is mainly for beefing up walkman style headphones. Wouldn't really work on HD600s in my opinion.
Buy cheap, buy twice!
I'm looking at it from a standpoint of extra drive for the cans as my soundcard on max output only drives the HD600's at moderate levels.
You may have some luck with it, but it's probably more suited to the much lower ohms of walkman headphones. I maybe wrong though! Do a little google search. Sure someone somewhere has done it!
If you are looking at the E7 may be worth considering the NuForce Icon uDAC2-HP
which is about the same price.. Havent heard either.. but I really like the NuForce Icon HDP (The biggest differences for me are the clarity, tighter bass and instrument separation)
Just tested this now using Modus Operandi with fairly low volume. (this was a pretty quick and dirty test)
I have HD650s and a Nuforce Icon HDP
Its actually really hard to do the test, as the mismatch in volumes when you are changing configuration can be misleading.. I tried to adjust for the same perceived vol
The difference between connecting the headphones directly and using the amp was subtle, the biggest difference was in "timing" ie. punch of the drums and less mud in the lower frequency stuff < 500hz.. hi frequency was a little brighter sounding with slight improvement in clarity on transients..
not sure wether I would drop the money on it though just for this improvement... If you need it to be louder then the amp will do it x100000
To be honest the biggest difference I got was going from analogue out to digital (using external DAC) mainly with the clarity and soundstage.. really able to listen to the reverbs being used almost as if you can walk through the sound picking out different bits you want to look at..
I may be talking bollocks though who knows? see if you can borrow one or try one out in a shop it is quite a personal thing really. I had a Benchmark DAC 1 a while back which I borrowed and decided it wasn't worth it for me.
I wouldn't recommend dropping extra cash on headphones before getting a nicer soundcard, especially if the headphones need a decent amp to work.. Personally I noticed that I messed up the bass on a lot of my mixes using the 650s without the amp (probably because i am shit) but with the amp the mixes translate much better.. (this could also be because I have gained experience making the mistakes before)...
A dac is what converts the digital signal in to the analogue signal which is then amplified by the amp. So in your case your Mac does the conversion, in my case I have a separate unit which does the conversion (and also includes a nice headphone amp)
cheers
a Soundcard is probably your first step. there are a million options (have a search around here) and plenty available second hand for not too much...