Anybody mixed speakers from different series?

[Woochifer]

Definitely do your listenings first!

B&W has a spotty history when it comes to adequately timbre matching the center and surround speakers on the 600 series. And from what I understand, the 600 and 700 series are also notably different in how B&W voiced them. If your local dealer will allow, you should arrange to borrow the center speakers from the 600 and 700 series, and make note of any changes to the timbral match across the front soundstage. Just make sure that you level match the center channel with a SPL meter.

With the surround speakers, I would suggest that you try both a set of bookshelf speakers and the dipolar surround speakers. I generally did not like how B&W designed their dipolar surrounds for the 600 series (I have not heard the current versions, which no longer adhere to the THX guidelines -- not a bad thing IMO). In the end, you might decide that you like the direct firing bookshelf speakers better.

If you got space and budget constraints, I would actually opt for a pair of 700 series bookshelf/surround speakers as the surrounds first before going to the center speaker. A receiver can create a convincing phantom center effect (as Kex said, limitation being how it shifts when you go off-axis), but a convincing surround effect requires properly aligned speakers off to the side. To me, the split surround effect is by far the biggest leap forward when making the jump from two-channel to 5.1.

[StanleyMuso]

I think I'll wait until the new series 600 become available and then arrange a comparison between the 700, the old 600 and the new 600, particularly the one with the FST mid. For rears, I'm thinking that the smallest bookshelfs from the 600 range would do - after all, they will mainly be reproducing special effects, and at this stage I don't intend going into DVD-A nor SACD - good titles way too hard to find - and I don't have the equipment to play them anyways.

[Woochifer]

I think I'll wait until the new series 600 become available and then arrange a comparison between the 700, the old 600 and the new 600, particularly the one with the FST mid. For rears, I'm thinking that the smallest bookshelfs from the 600 range would do - after all, they will mainly be reproducing special effects, and at this stage I don't intend going into DVD-A nor SACD - good titles way too hard to find - and I don't have the equipment to play them anyways.

Definitely give a careful listen before deciding how you want to go with the surrounds. Differences in how speakers reproduce the highs make a notable difference in the overall cohesion of the surround soundfield. With a lot of movies, indeed you can get away with non-matching surrounds because a lot of older movie soundtracks primarily mix ambient cues into the surround channels with not a lot of sound effects that bridge with the front channels.

But, with a stellar soundtrack like Master and Commander in which the sound effects are highly directional and equally tied in with the front and surround soundstage, a properly matched set of surround speakers make a huge difference in how immersive the experience is. Some newer soundtracks get repurposed for home theater, and those matching surrounds really help with those titles in particular, as well as with a lot of concert/music DVDs.

Personally, I would opt to go without a center speaker rather than non-matching surround speakers. Perhaps the newer 600 series would match more closely with the 700 series, so definitely weigh you options before assigning a lower priority to the surround speakers.