I've owned the S38's and the S-Center and they are very efficient speakers but the 8" woofers will take up alot of of your 80 watt per channel real estate. I would definately cross them at say 60Hz to a sub. Mine sounded great (but I had mine hooked up to an outboard amp) You should be fine with your current reciever just set to small and cross over accordingly.
The jbl s38's are very loud and clean sounding speakers. Sound and Vision did an article on them many years ago and they rated them very well. The good thing is that you can always find matching s38's on various selling sites (I see them on craigslist all the time) They are very heavy and well built speakers (except the fact the s38's have to be placed sideways instead of vertical like most bookshelves.
I loved mine and hated selling my set, but a co-worker really needed a set and he had done a lot for me, so glad somebody got some great use out of them.
and I agree the jbl's will blow those Bose speakers out of the water, your gonna love how much better the jbl's are compared to the Bose.