Hello,
More than likely, the LCR60 will work out better. While phantom, when the speakers are optimally positioned, can do a great job. The ideal is to have a 3 speaker front stage. It is only because your 704's are in a higher Series that I recommended at least trying it.
As to surrounds, if using the 704's in your HT, the DM 6000's will provide a much better sonic match. Furthermore, they should match really well with the LCR60.
I suppose it comes down to how good of a deal it is. Also, are there any Audiogon listings for used B&W center channels/surrounds that are 700 Series in your area?
Since you have and like the 704's, adding additional B&W's really would give you an excellent HT experience. Ideally, having matching tweeters should be the goal. However, if the lower Series you are looking at are a good deal, you can always sell them later and recoup much of your initial investment when upgrading to the matching center/surround to your 704's.
Cheers,
JJ