Hmmm. The HPA4 seems to have been well reviewed. I'll find someone who owns this so I can audition.
Although, I'll have to admit that benchmark products have never succeeded to impress me. Before I got my monarchy dac, i had got to audition the dac3 and the benchmark power amp. It was not worth the price the gentleman had paid. Detail retrieval was average and the seperation was pretty bad; (considering Ofcourse the price.). The dac3 headphone out was even worse but hey they never said it was a dedicated headphone amplifier.
From whatever material I read about the HPA4 in the past hour; the THX tech, the screen, preamp functions etc, the whole proposition makes me even more suspicious of its performance. I'd also factor in the THX 789 (which IMHO is just extremely overhyped); an amp I found to be the proverbial "bakri" amp for audiophiles all around the world who bought their units on the recommendation of Youtubers and so called "Reviewers" without even auditioning the amp (moment of silence for our bakris and bakras).
But I guess that's the trend in this buisiness. People wanna buy the stuff they saw not the stuff they heard.