Heck, I will jump in too.
I have HD600's. They are a bit overrated. Their bid sin is their low end is lacking. For their price, they should be better in that respect.
They are difficult to drive.
It also depends on your source and it's resolution. No point on an $800 amp if you have a mediocre source. Total waste.
It's also a waste to get an $800 amp using the standard HD600 cord.
To get good low end and full capability of the HD600's you must get an aftermarket cord, like the Clou's at Headroom. Expect $100 here.
I have used the Creek OBH11. Not that impressive, and the warranty card says England. Might have a problem getting quality repairs.
I have the MG HEAD OTL. Again, no USA repair facilities.
I have had the Headroom Supreme (new model). Less than stellar with the HD600's, but to be fair, it was with the standard headphone cable for the HD600's, and I think I would have kept the Supreme if I had the Clous then (I have them now, the DO make a big difference, to me).
I am going MG Head to head to the new Headroom Little. I have both right now. The Little can drive the HD600, with Clou cables.
But the Headroom Little is a fine little entry level amp. Better than the Creek by far. But it does NOT have high end sound.
Sorry guys. That is why the Little can be upgraded and Headroom sells more expensive amps. The Little is good for most people, but not if you want "audiophile snobby sound", which is what I like. The Little will not show the full capabilities of the HD600's. Neither will any SONY CD player, though, or most CD players. But I have two SONY CD units (my main one is a Rega) and they will sound ok though for the Little.
The MG Head sounds better when it comes to audiophile sound. A step above the Little. The difference is obvious. I have done some tube rolling too. I have no idea how the Little with Premium module sounds.
So. Back to the original posters questions. The Headroom Little at under $300 is the best choice there.
Then the MG Head OTL.
Then you should get the option cable, a CLOU or Cardas.
Perhaps a Little with a Premium module if you can afford it.
I use the Little now with Sony units (CD and MD) with HD570s in my bedroom, and the sound is very very nice. No match for my main system. But good.
My two cents.
If you don't want to go through the agony of getting the HD600's to perform like they can, sell them and get some other headphones.....a cruel statement but true. Headroom has some good stuff and a 30 day return policy.
Other headphones sound almost as good and do not require the gold plated amps.