This is my setup. I'm using 6 hifonics hfi 12d4's. Off a single audiopipe 15001D. Tuned high. Peaked at 64 htz.. And i'm sitting at 1.34 ohms. Just remember not everything is what it seems.
http://www.youtube.com/lovindatuba#p/u/3/jYIGw7PGyl0
Now I will say this much. I heard 2 of the vibe audio 15's in a ported box off about 1300 watts rms. They sound pretty loud. Low. And POWERFUL!!! but yeah. try that on for size 163.8 off less than 3k lol.
Interesting and very impressive, man! What vehicle do you have?
I know that so many variables do play a role in SPL and I'm not necessarily stating that it is absolutely impossible for the increase in DB. It's just very hard to comprehend. But on face value it should really make some sense being that he and you both are using almost twice the effeciency of the subs previous, and far less power. They fit the required specs and must like the box a lot better than the last subs did.
I come from nothing but trunk cars (mine at least), so I can only speak from so much experience.
After all these years my philosophical acceptance as to the "Universal Theory to Get Loud" is...
"If you have a small amount of air space to work with (i.e. trunk or whatever) the most efficient amount of cone area you can fit and the most power you can throw at 'em will yeild the best results. If you have a large amount of space; cone area, box design and tuning play a much larger role than just more power." At least that's what I think. If we quantified every single aspect to each specific vehicle, firing direction of subs, power, tuning... the whole 9, everything really is so subjective and relative to each install.
Very nice once again, and maybe one of these days I'll have something really loud too.