I am a big fan of using 12" top cabs if you have 15" subs. I find there's less 'mud' in the mix from the top cabs emphasizing the low mids by the larger speaker. If you have an 18" sub, them maybe OK, but one of the very best combos I ever worked with (albeit unpowered, but JBL make the PRX powered line, too) was the JBL SR series 12" on top of Mackie subs. Absolutely AMAZING power handling and throw, and more low end than some 15" cabs, definitely enough to hook up well with subs crossed over at about 150Hz...

Not cheap, but you get what you pay for, plus they have the benefit of being small enough to use on just about ANY gig, not just high volume ones.