Hey guys,
I can tell this thread seems kind of dead, but for what it's worth, I thought I might just put in my two cents.
I've got the HD598s and I've had them for over a year and I love them. For the longest time I always just had them plugged straight into my computer and I think they did sound really great, but as most people have said, maybe just a little neutral/weak in the bass and I always wanted to try them with a decent amp, but could never really afford it.
Finally though, I ended needing to buy a new amp for my speakers and I ended up with a Yamaha RX-770, which has a great reputation and, for $150 was a great find. As far as my headphones are concerned, it's great! I can understand that they probably won't have gotten as much improvement as some other high impedence headphones, but there definitely has been a great improvement to their overall dynamics and power, with the most noticeable improvement being in filling out of the low end. It has significantly improved the overall low end authority and has given thme great super low end extension. They still aren't 'bass-focused' headphones, but as they are at the moment and for the music I listen to, I would actually be apprehensive about trying something with much more bass, otherwise it would start to take over the sound signature.
The amps's a pretty meaty thing, putting out over 100 watts for my speakers at 6ohms, plus I think something that has also helped in producing great bass with these cans is the well-above average damping factor of around 240.
Anyway, those are just my thoughts.