Subwoofer

My speakers go down to 27 hz, but with the subs I'm hearing bass details that I've never heard before. My system is 2 channel only.

I think there's no question you will get deeper and better quality bass with a high quality sub and crossover...

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