Crossed over properly, they would work beautifully.
I currently run a NHT Evolution U2 sub setup with three B21's in a music only rig. Very pleased with it.
IMHO, these are speakers that, while literally dirt cheap, are not at all over matched when paired with a high quality sub. In fact, you do them a huge disservice by NOT going with a quality sub.
Having said that, you are still not going to get 105dB out of them, but kept within reasonable limits, no problem.