Because we were swapping through so many amps and cabs, we stuck with the two basses Steve brought along - his alder/maple Sadowsky JJ 5-string and his ash(?)/rosewood Mbass JJ 5-string.
The amps we used included Steve's three Genz-Benz heads (Shuttle 9.2, ShuttleMAX 9.2, Streamliner 900), Fender Super Bassman 300, Mesa/Boogie Titan V-12, Mesa/Boogie WalkAbout, Aguilar DB750, Kern IP-777 (through the DB750's power amp section), Orange Terror Bass 500, and GK MB Fusion 500.
A few of these amps did seem to prefer the 15/6 (Kern IP-777 and Titan V-12), and - when pushing a heavily overdriven tone - the Super Bassman and Orange seemed to really shine with the NV215. Note, we probably could have achieved equally good results with these amps pushing the NV115, but they seemed to click a little more quickly with the 15/6 and NV215, respectively.
That being said, the overall impression was that with most of the other comparisons, the NV115 (particularly the one with the newer Eminence driver) stole the show. I can see where some folks would prefer either the ML-112 or the Hathor, but IME/IMHO, the day belonged to the NV115. My initial skepticism that it would be too small (for a sealed enclosure) to get very loud or hit very hard down low was pleasantly dismissed.
Jim told me to not be afraid to push the NV115 with some serious power (which several of these amps can really deliver, and which the NV115 soaked up without complaint), but what really impressed me was how well they responded with the relatively modest powered WalkAbout. Even driving one 8-ohm cab, the WalkAbout was able to generate an impressive amount of volume. Two NV115's and a WalkAbout could cover a LOT of gigs, and the tone was glorious!
I would also like to point out that the Terror Bass 500 (set to 8 ohms, and cranking out 500 watts) into one NV115 was also amazing! Crazy volume, big tone, TIGHT lows, man, that was a rig that sounded a lot bigger than it looked!