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.

Thanks for info, one more question, would I need to employ filters on both main speakers (highpass) and sub (lowpass) for proper integration, or I can leave mains as they are and only filter higher frequencies on sub?
My speakers are 6dB down at 40Hz, I was thinking of connecting subs to 2nd output on pre-amp and employ filter on sub only, in order to avoid additional signal processing for main speakers.

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.

In most cases, adding subs will cost substantially less than going a model up with main speakers, and get you better bass. Also, since the subs are typically active, you are giving you amp a break with the subs.

What I did with absolutely stellar results is this. Get 2 very fast subs (I have JL Audio 113). Run 2nd output from your preamp into an external crossover (I use the cheap NHT X2). run the low pass signal into a SVS (or audessey - some unit), 2 channel sub equalzer in discrete stereo mode. Dont mess with your mains signal - run them at full range. Play around and do some in room measuremt to find the best crossover point for your low pass filter. Apply bass EQ to your stereo subs.

I am with Stanwal for the most part, though I haven't rolled any sub off as low as 25 HZ.

To me the integration is the different speed characteristics. A "slow" sub is very difficult to mate with speakers. This is the area that is most noticeable as well in listening to music.

I am suspicious when I see subs with 15" drivers advertised (especially if they don't cost a fortune $5K-$10K+, and to some degree even with those that do cost a fortune). A 15" driver is a lot of driver to move quickly and stop quickly.

In my book, the faster sub will be easier to integrate. I would rather give up some absolute bottom frequency extension to insure I get the speed I need (especially with your ATC speakers).