[devilmonk]

Looking to buy an amp for my Beyerdynamics DT 990's. So far, I've been using the Asus Xonar Soundcard. Gets to good-ish volume, but it never gets past a certain point. It has a smart volume boost for low audio files, but it adds A LOT of static. 
 
Also, I heard the headphone can get improved SQ with an amp. I thought an amp's only purpose was to bring out the volume for high OHM headphones. Can someone explain this a bit better? Still kinda entry level. Only owned one other expensive cans and that was the Soundmagic HP150's.
 
Budget is ~$100. If sound quality does indeed improve, not just get louder, then I'll up the budget to ~$200.

 

[Shaffer]

A Matrix M-Stage from the forum classifieds. Good luck.

 

[PurpleAngel]

  Looking to buy an amp for my Beyerdynamics DT 990's. So far, I've been using the Asus Xonar Sound card. Gets to good-ish volume, but it never gets past a certain point. It has a smart volume boost for low audio files, but it adds A LOT of static. 
 
Also, I heard the headphone can get improved SQ with an amp. I thought an amp's only purpose was to bring out the volume for high OHM headphones. Can someone explain this a bit better? Still kinda entry level. Only owned one other expensive cans and that was the Soundmagic HP150's.
 
Budget is ~$100. If sound quality does indeed improve, not just get louder, then I'll up the budget to ~$200.

 
Hopefully you disabled the motherboard's on-board audio, in the BIOS, before you installed the Asus Xonar?
Which model Asus Xonar sound card is it?

 

[devilmonk]

Asus xonar DG headphone Amp & PCI 5.1 audio card.
 
Yes, I disabled it. It has 3 modes - <32ohms, 32~64ohms, >64ohms. It wasn't exactly designed to run such a powerful cans, but it's gotten it to go to a moderate volume. 
 
I'm also looking to buy from a dealer, not second hand or used. Currently looking at Schitt Magni, Schitt Vali, and O2 amps.

 

[PurpleAngel]

  Asus Xonar DG headphone Amp & PCI 5.1 audio card.
Yes, I disabled it. It has 3 modes - <32ohms, 32~64ohms, >64ohms. It wasn't exactly designed to run such a powerful cans, but it's gotten it to go to a moderate volume. 
I'm also looking to buy from a dealer, not second hand or used. Currently looking at Schitt Magni, Schitt Vali, and O2 amps.

 
Any of the amplifiers you listed can easily drive 250-Ohm Beyers.
The Sound Blaster Z sound card ($65-$80) has a more powerful headphone amplifier and better DAC chip, then the Xonar DG.
Get the FiiO E11K Headphone amplifier ($60) and plug it into the Xonar DG.
You might consider selling off the DT990 and get the Philips Fidelio X2, as it's only 30-Ohm, it's very easy for the Xonar DG to drive.
Get the feeling the Fidelio is like an improved DT990.