I formerly owned Pioneer HPM-100 speakers, designed specifically to compete with the JBL L100/4310. I've heard L100s a lot, and have asked myself your exact question many times.
Unfortunately I can't do an A/B comparision since I no longer have the HPM-100s. But I've played the exact same stereo material on my Axiom M60s that I played many times on my HPM-100s, trying to determine are the Axioms better, just different, etc.
There's no question the L100 and HPM-100 were very impressive speakers for their time. With two 12" woofers (per pair), you definitely didn't need a sub. They were very efficient, probably equivalent to M60s in this regard. Overall my impression is my M60s are more accurate and playing the same material I hear things I never heard on the HPM-100s or L100s. OTOH those old speakers had a "kick you in the chest" impact that the M60s don't have (by themselves).
Augmented with my Hsu VTF-3R sub, the M60s are better in every way -- more accurate, better base, etc. JBL never published response curves for the L100, but I think other people tested them and they weren't stellar. It's the old controversy -- which counts more, good subjective sound or good numbers?
The NRCs research shows that a scientific objective approach to speaker design produces good sounding speakers. The L100 and HPM-100 certainly sounded impressive, and some people prize them even today. However given the choice I'd take my M60s over the L100 or HPM-100 any day. One caveat: if you're used to speakers like those old ones, you should probably have a sub, even with M60s.
Some related links:
HPM-100 pix:
http://www.fifartwc.rr.nu/~patrik/HPM-100
http://www.oldhifi.com/pioneerhpm100.jpg
History of L100:
http://www.audioheritage.org/html/profiles/jbl/l100.htm
L100 Owner's Manual:
http://manuals.harman.com/JBL/HOM/Owner%27s%20Manual/L100om.pdf 
