I've not heard the HM801, but the S:Flo has a very similar treble roll off which I immediately heard. I noticed it with my DT880 and my Earsonics and afterward, when performing RMAA tests, I realised it wasn't just hearing a 'smooth' nature. I'll not apply that too the HM801 any more than by saying it is audible meaning that it is an applied filter.
Some people will like that 'smooth' sound, but others will wish for sparkle and higher resolution up top. I DID say that if the S:Flo's GUI was good, it was smaller and free of artefacts induced by the silly touchscreen, I'd buy it. The HM801 has some other tricks up its sleeve: performance as a DAC, etc., which is nice.
But I've never even used a high-end amp that purposely rolls anything off, especially in the treble.