What's an inexpensive amp that will properly power beyer dt990/600?

The HeadRoom Airhead fits that calling perfectly, and its only $99.99!

And being that this is HeadFi for God's sake, I cant stop at recommending you something under your budget, without recommending something to you thats over your budget.  In that case, the Millet amp would be a great choice.  

You could get an OTL tube amp called the PHENIX G3 for $150. It drives the beyers very nicely and I believe it would power them better than the Airhead in this situation since OTL provides the voltage that high impedance headphones need and the Airhead can not provide that.

If you can't afford the PHENIX, the Airhead is a doable alternative. It's indeed a nice amp but it's portable so I don't know if you want portable or not. Once thing to keep in mind is that the PHENIX is a very compact desktop amp and comes with an external power supply.

I'm using my DT990/600 with the Fiio E9. More than enough power for them, and keeps the innate sound signature of the 990s. $130 alone.

If you're not a fan of the 990's brightness, the E9 won't be for you, as it is a relatively transparent amp and doesn't mask or hide the sound.

Tubes seem to play very nicely with the higher ohm beyers. If you could stretch the 150 a little you could get a little dot mkII delivered for 179.  I run a set of dt880/600's wiht a little dot mkiv and they are amazing.  It will definitly help to soften up the highs in the beyes a bit and would provide more then enough power for them. 

-chris