Slacker G wrote:How MANY 15"s?
Hmmmmmm , So are you comparing? 6 10's with 6 15's? Personally, I'd still take 6 15's over 6 10's. But I guess it comes down to personal taste. I would love the subsonic boom of 6 15"s beating your brains out on idle. 
Okay, sure, I'm using 6x10s. Which does make the comparison a little skewed. But...
The biggest thing I've notices with tens that handle the lows is they don't handle the power. I can find fifteens that handle a lot more power. But given roughly equivalent cabinets (2x15s vs 6x10s or 1x15 vs 4x10s) the cabinets with tens in them perform better, sound better and are (bonus!) generally easier to transport.
This is from side-by-side comparison.
But there's also the specs. I don't remember the exact numbers, but I recall that any one of the drivers in my 6x10 cabinet will reproduce an even volume at frequencies that the 15s start to roll off severely. Distortion characteristics for the tens are better as well...at both ends of their usable spectrum.
Fifteens are
not useless. Good quality fifteens do perform reasonably well. I can't be bothered with seventeen or eighteen inch speakers. Unless you are really driving them hard, they don't really perform well at all. Pure mud.
But it isn't just the drivers that matter. For fifteens to really perform, the cabinet needs to be large relative to the size of the drivers, IME. Tens can perform well in a much smaller cabinet. I used to understand the physics involved in that, but I haven't used it enough years that I've forgotten it all.
All this being said: a 2x10 cabinet is, IMO, too small a cabinet for anything other than practice. But it's a little difficult to be sure since my is rated for 350 watts and I'm pretty sure I at least once drove it beyond it's rating. The drivers have never really been the same since. My 6x10 is rated at 800 watts or 1000 watts depending on which spec you read (I believe the 800 number, personally).
Apples to oranges? Well, sort of. Except that you really need to compare the cabinet as whole. The "speakers" by themselves are only part of the system. I've also taken drivers which sounded great in one cabinet and put them in a different cabinet where they sounded like crap and vice versa. Oh, yeah, and there's the quality of the drivers to consider.
If you want a valid comparison, you can't focus on the drivers alone.
You don't work music, you play it.
Discipline is not the enemy of fun.