What does a dedicated server do for the quality of the music?
Lower noise,
if there's any. Depends on whether your computer has any electronic or fan noise; also a more dedicated and usually smoother interface (ie, not necessarily iPhone smooth, but in the sense that you're not switching between apps and windows, unless you get distracted using other apps on the device running a remote app) with lower power consumption, faster start up, etc. If you're not running any intensive apps while listening you
might be able to get away with just your Macbook Pro.
What would be a better improvement, getting a low sensitivity headphone (suggestions?) or the Valhalla 2?
The Valhalla2 has a bit of an improvement in imaging on high impedance headphones over the Asgard 2, which to some has a tendency to make the HD6xx line worse (personally, not having the cymbals too far out to the flanks is just "proportional imaging,"
not simply just a smaller soundstage).
A low sensitivity headphone on the other hand is not by itself a feature that you'd want to have, because that means you're going to need more power. Even though the Asgard has lower power output at high impedance, the higher sensitivity of the HD6xx line means that you don't really need that much more power than what the Asgard can spit out at 300ohms. Amp upgrades past the Asgard, other than for hard to drive planars, are more for even lower noise and distortion (or better imaging) than for more power. I can't imagine why anyone would want a lower sensitivity headphone for that feature alone.
If what you want though is an altogether different sound signature on the headphones, aka "what compromises the engineers chose to make, because totally flat response designs can not be achieved with current technology," then the lower sensitivity would be an acceptable compromise. If it's not too low and the impedance is also low, then the lower sensitivity can be offset by the higher output of the Asgard at the lower nominal impedance. It's not that simple to just toss around any of these though - we need an idea as to what exactly do you not like about your system, if not particularly the HD650, to figure out which headphones might be the most suited for your preferences.
