PS Audio BHK versus Herron VTSP-3A(ro2)

The volume difference you notice between the two pre's may be due to the differences in how they each handle gain.  I think the Herron is fixed at 14dB and the BHK uses a switched gain adjustment linked to its stepped volume controller.

If the Herron is SE all the way through and the Herron preamp provides 14 db gain then that explains the difference in level as the BHK in SE only provides 3 db gain so turn it up.  Are you connecting the DS Sr. to the BHK balanced and the Herron phono SE?

With the Herron, I'm using SE all the way.  With the BHK, its SE from the phono pre and balanced from the DSD.  I've noticed the Herron streaming sound is warmer and the BHK sound is more nuanced and detailed. 

Whenever I change a component, I play "East River Drive" by Stanley Clarke, "Crystal Morning" by Dave Grusin, "Since I Fell for You" by Brad Mehldau, "Imagine" by Randy Crawford, and "Insanity" by Gregory Porter / Lalah Hathaway.  Each has loud passages that the Herron tamed at my normal listening level, the BHK not so much.

I'm using a Matrix X SPDIF II between my Innous Zenith MK II and the DSD. WW Platinum Starlight 7 USB to the Matrix and a low budget HDMI cable to the DSD. I have Tannoy DC8Ti speakers.

I'm using a NAD M22v2 power amp.  NAD and class D don't get a lot of love on agon, but it sounds fine to my ears with the Herron. I want a warmer sound, so my next upgrade will be a tube power amp or mono blocks.

Before the BHK, I auditioned an LTA ZTOL40 and it sounded wonderful.- but it doesn't have balanced connections. I'm planning to move my gear from between my speakers to a side wall and will need a long interconnect to a power amp or long interconnects to a pair of monoblocks.  I'm thinking balanced is the best way to go.

Vol matching when comparing gear is important. Like you said, you can get the same sonics when turning one up more. So use a spl meter to match and then see what you like best. 

All too many A/B listening tests are skewed by mismatched volume levels.  As just said, unless the spls are pretty closely matched, comparative listening like this won't tell you too much.