The Topping Pre90 measures extremely well and my subjective impressions are it is amazing. Somehow it adds clarity to the sound. I really don’t know how to describe it but it definitely improved everything. Even vocals were at the beginning strange because somehow they were more dispersed than when using the integrated preamp of the M400. After extensive listening what I’ve noticed is that vocals seem be more upfront now, like closer to the listener.
Only caveat I can tell is that the remote control is truly dismal. It was an afterthought, and I cannot understand how such a great sounding equipment, not cheap, they didn’t include something even mediocre, because it is worse than that. Basic problem is you need to point directly to the unit. You get used to it, but with today’s tech and remotes, it is very annoying at the beginning. To the point I considered sending it back just because of this. Only reason I kept it was because when I went back to using both my home theater and the M400 without it, I didn’t like them.
An alternative I haven’t tried is the Schiit Saga and Freya. The Saga doesn’t have balanced inputs, which you might or might not need. That one is also cheaper, and supposedly more transparent.
Hope this helps.