The idea of driving HE-6 using speaker amps has never even occurred to me, but if what you say is true, then I suppose I haven't heard the real deal out of the HE-6 yet. However, I can't possibly imagine having a pair of huge monoblock power amps in my desktop headphone system - not because of the price, but because of their sizes. If I had enough room for those, I may as well listen to actual loudspeakers.
Please excuse my ignorance, but how do you use speaker amps to drive headphones? My first assumption is that you must have used a dual 3-pin XLR balanced cable (for each monoblock), but how did you plug it to the speaker outputs of the UPA-1?. Other than some DYI work which I'm not capable of, is there, say, a "speaker outputs-to-3-pin XLR" adaptor to make this possible? Or any balanced aftermarket cables for the Hifiman that can directly plug into the speaker outs from both monoblocks?
You have a fishtail made to attach to the 4 pin HE6 cable and you can use a 50W intergrasted amp like an NAD which wont kill you space wise. Speaker amps IMO are the only way to go. The Pioneer sx980 was 80W per channel and it put out 15W into 50 ohm HE6 and it was the best I heard it when I owned it. Still sorry I let it go.