I never could make myself like the HD-600/OBH-11 sound. I've heard alot since then, but going from memory, the sound wasn't very balanced, with too much emphasis on the low end, the top end didn't extend as much as I'd like it to, and their wasn't really the air between instruments, I remember a rather soft sound, lacking in detail as well (maybe I'm spoiled with the gilmore/ety combo).
IMO your $350 would be MUCH better spent elsewhere--though I only heard it breifly before I sold it (the Corda HA-1 took my attention), and my ears were even more virgin then, the headroom little sounded kinda nice with the senns.
Today there are quite a few options at your price point which I think would be a much better bet the the creek, some examples are as follows.
Gilmore Lite (pre-order price of $250) - haven't heard one but I love my v.2
Meta42 - I heard one with specs unknown to me ( I think it was williamgoodys) at the NYC meet, it deserves the hype
Corda HA-1 MKII - More expereinced ears then mine swear by it, and the HA-1 was already a pretty nice little amp
Melos SHA-1 - I only briefly listened to a melos and it was quite the expereince... it was boozebuttons carlos modded frankenmelos, and it made my senns sing like I didn't know possible. Of course this is probably alot better then a SHA-1, but they've pretty much been praised for a while.
MG Head - My expereince with these was breif but pleasant, definatly worth investingating
$350 is a nice amount of money to play with. Do a little reading and I'm sure you'll find something to your liking.
Hope this helps,
Fred