The same output levels at different frequencies will probably sound and feel different. Also, if one sub has more distortion than another at the same level, it will sound louder. A perfect example of this is at the local competition I went to earlier this month. Everyone outside my car was like, "I can't hear his sub. Is it even on?" And yet I pulled just under a 144. In fact, my stereo doesn't sound that loud compared to others because it doesn't produce distortion. But it still has huge impact.
How much are those subs going for that you were looking at? Audiobahn does seem to make some decent subs. But they always seemed to be way overpriced. I'm sure we could recomend another sub for much less that will be able to get louder. A perfect example of this is Adire's Tempest. Marshal Joyner used to use this 15" sub and consistantly hit 152 dB with only 600 watts. And it was tuned low so when he turned it down, it had awesome SQ. Not bad for a $150 sub.
What kind of vehicle do you have? How much power? How much room are you willing to give up? Is it going to be just for SPL comps, or do you want it to sound good for daily driving as well?
Sony CDXC-90 -- Sony XDP-4000X -- Adire Koda 6.1 -- dual Adire Brahma 10's isobaric -- Arc 1500dr -- Soundstream Reference 300
"We got tired of the "Xmax" wars, so while everyone else is playing with bows and arrows, we went nuclear." Dan Wiggins