I think it's better to buy a nicer headphone with a cheaper amp than a good amp and a cheaper headphone (within reason of course). The reason is that the biggest difference in quality seems to correlate with the headphone. I noticed a difference between my Total BitHead and O2/ODAC, but it wasn't nearly as big of a difference from going from a DT770 LE to the DT880 or the DT880 to the AKG K702 65th Annies/HE400, etc..
unfortunately when people get the cheap amp, they don't always upgrade in the future, and then they're always bottlenecking the headphone.
and theres a ton of those threads with people buying LCD-2 or HD 800 or whatever and then asking for an amp under 150$