Go for sub or larger speakers?

A lot depends on room size, I think it is easier to get a speaker/sub combination to work in a smaller room. But in the end it is a question of personal taste. I believe most of the integration problems arise from crossing too high or playing the subs too loud or a combination of the two. I haven't had much trouble integrating mine.

There are two schools of thought on filtering. One believes in using an electronic crossover and rolling off the main speakers. This undoubtably allows the system to play louder. The other, of which I am a member, believes in running the main speakers full range and using the subs to supplement the bass. This avoids the addition signal processing you mention. Subs like the REL, which I also use, can be driven from the amp outputs as well as line level, this allows them to track closely the behavior of the main speakers. I start rolling my subs off at 25Hz on the top end, when I set the level correctly you can't tell they are on. Just adds body to the sound; does this even if no low bass is present.