Subwoofer

At what frequency do you crossover ? If at or above 27hz you have the subs plus your speakers doing low bass duties ? I tried a sub crossed around 30 hz with some Thiels cs7's which go down to 23hz  and it sounded like they were competing with each other , was not clean sounding but i did not use an external crossover . I am unclear on what frequencies your speakers are handling . I would like to know how that gets setup . Seems like it would be beneficial if less taxing on amp/speaker relationship . 

Getting bass right is vastly more difficult than getting higher frequencies right. You simply have to move more air and deal with the interference issues attendant to the omnidirectional nature of low frequency information. Powered subs are purpose built for the first job and allow placement flexibility for the second issue.

I think subwoofers are almost always just a better mousetrap

YMMV

I explained it and some of you still don't get it.  You're not extending the low end, but making mids and treble better....also you change the acoustics of the room by evening out the "bumpy" low end.  Add the sub...or 2...or 3 

My experience with a sub is that even if your speakers have good low end response because the sub is going to be placed in a different position in the room, you will get more pact and generally a broader soundstage 

I have experimented with several different crossover settings and have found that 90 hz works best in my room.

stringreen,
I agree that adding subs has significantly improved the mids and treble