your reciever will be able to handle the 4 ohm load of the m80s.
the 704's are relatively new. i dont know how many people have heard them. What i have heard is that they are good speakers, but that they sound similiar to slightly better then the CDM7 series. i think B&W is trying to revitalize their speaker line without besting their 800 series. have you heard the 704s?
also, how big is your room?
finally, check out
http://www.axiomaudio.com/boards/showflat.php?Cat=&Board=first&Number=12763&page=0&view=collapsed&sb=5&o=0&fpart=
you can find someone who lives near you who might be willing to let your listen to their speakers
the m60s and m80s sound similiar, so look for owners of both
