I'm thinking what made subs so much louder back in the day is nostalgia. and inaccurate mics.
If you take a pair of old Kicker Comps from the mid 90s and hook them up to a 300w amp, sure it will sound good and get decently loud. I installed a pair in my friend's S10, in fact. They weren't ridiculously loud or anything, though. Same story with another friend of mine. He has an old S10C he bought off me powered by a 1989 Punch 150. It's a nice sub and amp combination, but nothing incredible. No doubt, older systems had to be more efficient to achieve the loudness they did, though.
I'd be willing to bet money that you can make a newer 10" louder than 2 old school 12's though //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif