Any Def Tech speakers with the same size drivers as your BP8's would work great in a 5.1 surround setup. I am not familiar with the BP8's, but it should not be too difficult to do a little research to find the 7000 series and which CLR center speaker that have the same size drivers.
Regarding not using a dedicated sub--even with built in subs in the main front speakers, a dedicated sub is needed. When I purchased my 7000 mains, I figured I could retire my old PowerField 1800 sub and did do so for a while, figuring that the built-in reference supercubes would suffice. One day I was tweaking and decided to connect up the old dedicated subwoofer and found the difference in LFE to be as much as the difference between night and day. IMHO, if you wish decent LFE, you will need a dedicated sub perhaps in a wish list for the future.
In retrospect, I would rather have my 7000's or a pair of 7001's without ANY built-in subs (would be VERY similar to BP30's) and then have dedicated stand-alone subs. YMMV
MikeSp