New 515= Altec - 515 - 15" Loudspeaker
515g= 515G
I am seeing that the highest recommended crossover frequency is 2500Hz. I would assume that your multicells will come in much lower than that so an expedition into experimentation with lower crossover points would net good results.

You state that you're saving for a Mac setup but if you could squeeze less than a hundred out of somewhere, the Behringer CX3400 might be an efficient use of capital. I use one with 416b's and 811's and it does a pretty good job.