I have it on good authority that HE-6 sounds very good out of the Project Ember amp from Garage1217. From experience, I can say that the Project Polaris amp (also from Garage1217) also does a good job, although, I don't have any high-end gear for comparison. Likewise with the RJM Sapphire headphone amp, although this is only available as a DIY project. I've got several class-D amps that sound good, but I think the aforementioned amps sound better to my ears. Of all these amps, the smallest is probably 15 cm x 10 cm and none are bigger than 25 cm x 20 cm.