How are you going to use the headphones? Desktop/Mobile?
I've spent many hours pouring the internet for a reliable, cheap headphone AMP, but there aren't many worth mentioning under $40 that aren't mobile and can be used in a desktop setting (most mobile headphone AMPs can't be used while charging)
Desktop Recommendation
$65 - SMSL sApII Headphone Amp
$33 - CMOY Headphone Amp
Mobile Recommendation
$24 - FiiO E6 Headphone Amp
I personally settled on the SMSL sApII. I couldn't be happier. I use it with both the Sony MDR-V6 and ATH-A900X, which are both relatively low impedance headphones like yours. The cheaper CMOY is okay, it gets the job done.
If you're going mobile, just get the FiiO E6.