Go for sub or larger 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).

For 2-channel audio, I prefer to avoid a sub. I'd rather that a top-rung speaker designer design my crossovers, than me.

I use the hardware Edorr identified, but in a different configuration. I filter the main signal with the NHT X2 and run the bass signal out of the X2 into a Velodyne SMS-1 room EQ unit, because:

My current room, like every other room I've measured, gets very ugly below app. 125hz. I can (and do) fix it down to app 75 hz with bass busters. Below that, active bass EQ is the only workable solution that I've found.

If you run main speakers that have any real output below 75hz, it will IME be very difficult to get seamless integration of the subs and mains because the main speakers are contributing very lumpy bass until their roll-off. If you filter the mains to remove bass below 75hz (or at whatever frequency your treated room dictates) you can get smooth response from the mains, smooth response from the EQ'd subs, and a neat hand-off that will hard/impossible to hear (for me anyway).

If your main speakers have little to no output below 80, 90, 100hz, (i.e. Sunfire cinema Ribbon Monitors)you can probably run them full range and "snug up" the subs from below. Otherwise, I prefer to filter the mains.

As Stanwal, noted, there are 2 schools of thought, and this is the school I'm enrolled in. Others take the alternate route.

Two more thoughts:

1) "Fast" is a descriptor I dislike for subs. People tend to think small drivers are fast, and - IMHO- that's not the case. I think "highly damped" probably communicates the idea at hand - tight bass with little overhang - but this characteristic is not related to driver size.

2) My set-up does put the NHT between my ARC LS-25 and my ARC 130SE, but it keeps the Velodyne SMS-1 out of the main signal path. The NHT seems to be benign (to my ear at least) but the Velo has no business in the main path.

Marty