Lucky for you the HE-400 is not picky at all with amps. You should come into the HE-400 thread, people would be more than happy to help. Some off the top of my head that are reported to sound great with the HE-400 include:
-Schiit mini stack (Modi/Magni)
-Little Dot MKIII
And none of those (with exception to the Lyr) will burn a hole in your wallet.