I have an HP-100. I rarely use it as I find it excessively colored for my taste, although I enjoyed it for some time.
I digitized the output from the line stage and from the "tape" output and they sounded very different, so I spent some time tracking the reason down.
The HP-100 uses a surprisingly low current N-FET follower and a tantalum bead cap for the line output. This combination has a very high and very non-linear output impedance and produces lots of even order distortion.
The end result is that some music genres and individual recording sound pretty good, but others not so much- as I said a very non-neutral sound.
However, swapping out various tubes and hearing the character changing was always fun, if, inevitably, unsatisfying.
Hovland’s propensity to use divining rods to select components and other eccentricities was always a source of amusement to me, and indeed some embarrassment as, being a EE, it all seemed a bit too, well, mystical for me to accept.