BEST SPEAKER THAT COMES TO LIFE AT LOW LISTENING LEVELS

I’m not sure you can do better for low volume listening, as electrostatic speakers are known for their resolution at low levels (mainly due to the lack of mechanical hysteresis - getting that heavy cone up and running - in dynamic drivers) and your Martin-Logan’s are definitely fantastic electrostatic speakers.  Panel speakers, like yours, are very nearly line sources whose sound pressure decreases 3dB with each doubling of distance (the inverse law 1/r) where as point sources (cones, domes, etc) follow the inverse square law 1/r*2 where sound pressure falls off by 6dB per doubling the distance.  I think maybe some EQ options that others have mentioned might be the best solution. A Schiit Loki+ 4 band EQ at $149.00 between your preamp and power amp would handily do the trick.  It’s totally transparent, easily bypassed, and made in the USA.  The Lokius at $299 takes you to 6 bands and balanced in/outs. The Loki Max at $1499 adds remote control to all the controls. Enjoy your great system!

JBL 4349.

You have a very nice rig.
I’d start with a Schiit EQ. You can use it like a variable loudness control. The top of the line even has remote.

Run a 1/3rd octave tone generator on your system along with a db meter and you'll likely find out "elliot" is correct and our ears are much more sensitive to tones from around 600hz to 3khz and even if your speakers measure flat they'll sound deficient above and below the midrange. The old Yamaha receivers did have adjustable loudness, but if you don't want to mess around with tone controls and decide to find speakers that use the Fletcher munson curve.There's several brands that design their speakers with that curve in mind and they get great reviews. Raidho is an overpriced example, Vienna Acoustics is a more affordable option that should compensate well for low volume listening. 

Omega Speaker Systems Super Alnico Monitor.