[SoulSyde]

Here are a couple of alternatives:
http://www.headfonia.com/dr-dac2-dx
http://www.headfonia.com/audinst-hud-mx1-dac
 
Both are very inexpensive with pretty descent reviews.
 
I would recommend not doing the all-in-one route.
 
On the cheap you could do the following: 
Matrix M-Stage for the amp: http://www.ebay.com/itm/Matrix-m-stage-Headphone-Amplifier-/260535704524#ht_9096wt_1151
HRT Music Streamer II for the DAC: http://www.google.com/products/catalog?q=HRT+Music+Streamer+II&um=1&ie=UTF-8&tbm=shop&cid=11566938663836358933&sa=X&ei=RK2TTri8Eanf0QHp2rDHBw&ved=0CEcQ8wIwAQ
 
The opamp of the Matrix is upgradable which is a nice feature.  Having each as a separate component allows you to upgrade one at a time to better equipment later on, instead of having to purchase a new DAC and Amp if you decide to upgrade.  Just my $0.02. 

 

[coolguyalex]

In the exact same position as the original poster. My current phones are AD900s so I'm looking something a step up from these. Headphones I'm considering for the HDP:

Alessandro MS2i
AKG (K701/K702)
Beyerdynamic DT-880 (250ohm/600ohm)
Grado SR325i
Sennheiser (HD600/HD650)

I'm leaning towards the Senn's at the moment. Also considering getting the Schiit Valhalla in the future but I'd like to see how the HDP goes on its own first. 

 

[SoulSyde]

The Senns would be a great pair with the HDP, but (jeez) you have a pretty broad assortment of headphones there.

 

[coolguyalex]

Yes you're right, I'm just not really sure what 'sound' I want yet. It's hard narrowing it down to one pair, I can see why so many people have multiple pairs. One day..

 

[SoulSyde]

That's a good game-plan.  Instead of spending $1,000 on one pair of headphones you can own a few that are $300/ea.  I do think you need to figure out what type of sound signature you are looking for before you commit to one of those purchases.  Each had a different sound signature, some greater than others.

 

[coolguyalex]

Well I suppose if I had to choose I prefer a warm or darker sound. From what I've read, the Senn's seem to fit into this category particularly the 650s. My only concerns are the comfort level and build quality but I am willing to compromise if they sound as good as others have said.

 

[SoulSyde]

I owned the HD 600s and they were amazing headphones.  I only sold them recently to fund a high-end IEM purchase (my newly acquired SE535s).  You may also want to consider the HD598s, as you can get them for cheaper and they sound very close, just not as detailed.  All of the Senns have a nice laid back, warm tone to them.
 
If you're starting to "move up the ladder" in the headphone world you way want to consider buying a few at the middle-end of the price spectrum so you have a few different cans to go to.  For example: I may go to the Grados when I want something with bright highs, and I may go to the Senns when I want a laid back, non-fatiguing approach.  So instead of picking up a pair of HD650s you could get the HD598s and a pair of SR125i's.  Just food for thought.

 

[coolguyalex]

Definitely will consider that. Thank a lot for your help, I'm still very new to this as you can tell. 

 

[SoulSyde]

No problem, glad to help.  

 
Keep in mind that my opinion is just that, but you'll find out what headphones work for you over time.  Part of the fun of this hobby is learning the gear.  I'm just a big fan of moving up the ladder slowly.

 

[UteroiD]

I use Sennheiser hd600's with my HDP and i think it's a great pairing.  HDP drives them easily, I usually have my volume up a little over half.  I listen to alt rock/classic rock/jazz/classical.  I also tried it with my cheaper shure earbuds and those sound great as well.  

 

[curtisinoc]

Getting my HDP delivered tomorrow . .  .  Gonna pair em up with my Ultrasone Pro 900 (recommended by numerous Head-Fi'ers).  Can't wait!  Hope it's a good combo.
 
Update:  Sweeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeet !!!

 

[Kobra]

I'm using my HDP with a pair of K701. I like it a lot and would recommend that combination to anyone who's after detail, clarity, transparency and neutrality. The bass is well defined without being either thin or boomy.