Adding a sub - what will I loose?

In my opinion (gotta qualify here) you have nothing to lose and everything to gain. As long as it (or they in my case) is properly integrated it can be a wonderful addition. My JL Audio subs won't work the way you're going to do it but if you're missing bass you can complete your sound if you pull it off with the right sub(s). When it is done right you never know a sub is present but the bass isn't missing when it should be there. My subs get the full range signal and my NAD M3 integrated can send either a full range signal or use the built-in bass management feature to limit the bass signal sent to the mains if I choose. Works like a charm.

Look at Rel subs with the Neutrix cable. Hooks to your speaker cable jacks and works wonderfully.

I would go with the Wilson Benesch Torus Infrasonic Generator.. There is no better way to get those low notes at electrostatic speed.. It will cost you about $10,000, and it is much better then the Rel..

or you could ask yourself a couple of questions?

1. Does my system totally involve me with its realistic and natural sound.
2. Is the presentation relaxed enough to allow me to listen for long hours.
3. Does my system sound pure? Or is it a little blurry?
4. Am I looking for more bass because I really can't answer yes to any of the above and I don't know what to do next?

Something to think about..

Jim

I'm with Bizango1 - it'll probably be a big gain. I run my mains the way you plan to, with a high-pass filter (in my case 60Hz). I go further and bi-amp the mains - I run a full-spectrum direct signal to the midrange-tweeter sections, and high-pass only to the woofers. I use a y-cable to split the preamp signal. The improvement in my system overall is amazing. I'm very happy with the result.

10 Grand for a sub,lol......more power 2 ya