In the price range of the Emotiva I would think a Garage1217 Project Ember II with the right tube may get you to where you want to be with the treble. I haven't used the Project Ember with the HE-6 but Garage1217 touts that it will run the HE-6. I would contact the guys over at Garage1217 before you buy and tell them your needs and have them recommend what they think will work. They make top notch products from what I've seen.What's are some amps in the same price range that you think would be better?
http://www.garage1217.com/garage1217_diy_tube_headphone_amplifiers_004.htm