If you're good with circuits, you can build your own. A lot of DIYers build their own headphone amps, and it wouldn't be hard to just dump 6-8 of them in a rackmount case. They range in price for parts from $20 each to several hundred $ each, but even the $20 ones are high quality if you build them correctly.
If you don't want to pay $300, you may want to look into this. Otherwise, it would save a lot of time to buy it.