I have been chasing the low volume thing for a while and have been through a list of speakers over the last few years. While the speakers you choose obviously make a big difference, I have found a huge factor in improving my low level listening is reducing the noise floor. I am strictly digital, and I keep finding ways to clean up the signal. With each improvement I hear more bass and better articulation of the entire spectrum.
As for speakers, I don’t feel that the sensitivity rating has much to do with how a speaker plays at low levels. As you have found with the JBL’s, the woofer just doesn’t wake up until you get to certain dB level. Keep in mind those sensitivity readings are typically taken at 1kHz, and do not reflect lower frequency response.
The best low volume speakers I have found to date are my current FinkTeam KIM’s, not overly sensitive, but extremely well engineered, and an easy load impedance-wise.

