I owned the Harbeth SHL5 Plus and earlier versions of the SHL5 and they all sounded great at low volumes. I actually bought them for that reason. The issue is more likely to be your amplifier. Harbeth's, when used with SS amps like Ayre, Aesthetix, Naim, Musical Fidelity and many others excel at low volume.
I've have never heard a Harbeth speaker that sounds good with any tube amplifier though many others will disagree. If you like your amp you should work with a Line Magnetic dealer to find the right speaker for your requirements. You might also find a dealer who would loan you a SS amplifier to try in your system to see if the Harbeth's might actually meet your low volume needs.
Good Luck!
I've have never heard a Harbeth speaker that sounds good with any tube amplifier though many others will disagree. If you like your amp you should work with a Line Magnetic dealer to find the right speaker for your requirements. You might also find a dealer who would loan you a SS amplifier to try in your system to see if the Harbeth's might actually meet your low volume needs.
Good Luck!