If you are going to rely on the internal crossovers in the amp, I would look at something with a high order low-pass crossover for the sub channel... such as 18 or 24 db per octave rather than 12. It may help keep the illusion of bass up front/ enhance your front sound stage and overall sound quality.
If you are of the school of thought that some watts can sound better than others (I'm not), USA amps has a five-channel which is not bridgeable... most amps today have a built -in inverter circuit in one channel only to allow them to be used in tri-mode... what this means is that one channel is not the same as the other and has different parts. Thank Fosgate for this... they made it popular... 99.9% of car amps today are all clones of Rockford Fosgate. Would you use two different component speakers for left and right? Do you smell what I'm cooking?! Based on the fact that it is not bridgeable, it probably does not have an inverter in one channel... but you would have to verify it with a tech at USA Amps to be sure. Bottom line, if this theory holds true, the USA 5-channel could be one of the best car amps on the planet.