its said the sx is supposed to be more towards spl, but thats relative to the RE and the SE series. the sx is definitly more musical compared to true spl subs like the mt,btl, mojo, etc...
you CAN make spl subs sound pretty **** decent with the right box, right install, and right tuning. my btl sounds amazing on music. in your case though with only 2x mcd1000's then you're better off with the sx's. you could put 4 mcd1000's ona single mojo and run it daily. definitly go with the sx's.