It isn't the size of the speakers (10", 12" or 15"), as much as what went into the design of them. I used to play a Garnet BTO with two Altec-Lansing 2-15 cabs that belonged to my bass player. It sounded really good for six-string. Those speakers were meant to be full-range drivers, and one of the industry's dirty little secrets is the fact that, at least until recently, bass amps were no different from guitar amps. The old Fender Bassman was bought by more guitarists than bass players. And if you ever get a chance to plug into a classic tube Ampeg SVT with an 810 cab, pack a few sets of fresh underwear for the occaision! :D

These days I have an Ampeg VT-22 combo (2-12) and an Ampeg VT-40 combo (4-10). I'm replacing the VT-40's four CTS 10's with one WeberVST ceramic California 15, and building a new cab for the combo. My research indicates that I should get better clarity across the spectrum, and less cone breakup.

To get back to your question, speaker cabs should be auditioned the same way as the rest of your gear: ears open, eyes shut. There are too many variables, in terms of guitars, pickups, string sets, amps, amp & effect settings, etc, etc, etc, to be sure of getting the same kind of sound as someone else. And then, even if you were to invade a Pantera gig, and kick Dimebag off the stage, when you played his gear, set up his way, you would still sound like [u]you[/u].

You're looking to drop a pile of coin on a piece of gear - you're entitled to 'test drive' some stuff before you lay your money down.