Yamaha DTXpress/DTXplorer/DTXtreme

The MMX4 should do the trick for your intended purpose. The question 
is the line level of the output: Normally, this line would run to a 
power amp where the signal could be boosted. Since the specs don't 
say anything about a headphone jack, I would wonder whether the 
output signal alone is strong enough to power your phones.

Stephen