depends on how you have them connected to an amp and wired..
if you can feed the l5s the EXACT max of RMS watts they can handle, all 4 to their max rms wattage, in a ported box that meets their specifications in cubic feet and such and tuned to around 30hz
they will still not be louder than the XXX15s, but you have to have BOTH of those 15 inch subs wired from an amp to receive their maximum amount of watts each, in a ported box to fit their specs. and tuned to around 30hz. those subs will pound, their 8500 rms watts a peice, means you need a/an amp(s) to give off 8500 watts a peice to each, i think one will pound harder than the l5s..lol more rms in a single xxx than in 4 l5s
but to power the 2 15" xxx.. its going to be quite expensive, and for all the upgrades your gonna need to make.. the 4 l5s will be cheaper in the long run... the xxx's also might tear apart your vehicle, and last i knew they didnt make em anymore..