They're all valid and very sensible questions.
The first thing to consider is performance. The year-on-year performance gains in speakers makes glacial movement look speedy
Seriously though, good speakers are good speaker, whether they're a model you buy today or from 30 years ago. The 602's sit somewhere in performance around or a little above the Zensor 3. There isn't a modern day B&W equivalent. The 686 s2 @ £350 is about equal on price when adjusted for inflation, but physically much smaller and far less sensitive. The £500 685 s2 is closer in spec though still smaller, following the fashion trend for more compact speakers. So the 602 s2 is a good performing speaker even in modern company.
Given the above, then the answers to your questions about the cost of building a full system take a slightly different direction than you were perhaps expecting.
With all due respect to Dali, I think it's fair to say that B&W is in a different league. B&W makes some seriously high performing speakers with equally stratospheric price tags to match. Those products aren't about show though. They're a test bed for new ideas that eventually filter down to the entry-level models. B&W's Nautilus tweeter is a good example. The first outing for that technology was back in the 80's prototype for what became the Nautilus speaker unveiled in the early 90's. More surprisingly though, another B&W instant icon - the yellow Kevlar woofer - was first developed as far back as the 70's. Here we are, four decades later, and the Kevlar woofer and Nautilus tweeter feature across almost the entire range.
Given the performance of the brand and its products then it's perhaps only to be expected that the price of a basic 5.1 system will be more expensive than the Danish firm's speakers.
If I was in your shoes and seriously considering the 602 s2's then I'd be looking at the classified ads right now. The 600 series has been around since the late 90's and has been consistently high profile, so there's a decent amount of used product in circulation. Knowing now what you do about good quality products standing the test of time then I'd have the confidence to look back at 600 series centres and surrounds as a viable route to building a damned fine surround speaker system. It'll still be expensive though; it just won't be anywhere near as expensive as buying new.
One big plus in your favour is fashion. There's a continuing trend for smaller and smaller speakers. Just like the trend for thinner TVs eventually lead to crap sound from all of them, the same will occur with speakers. Technology can't beat physics. Tiny cubes don't perform as well as big boxes. So while big boxes are out of fashion then this is your opportunity to capitalise on a buyers market. I'd be eyeing up CC6, HTM61 and 62, CDM SNT and HTM7 centre speakers.
Surrounds from B&W is something more of a challenge. I make no secret of the fact that I'm a big fan of diffuse rear surround speakers. These are dipole/bipole speakers that create a sound wash at the rear to hide the localisation of the speaker position itself. IMO, the smaller the room then the more important it is that you can't tell where the rear speaker are located just by sound. The rub with these is cost. Diffuse rear surround speakers use more drivers and they're made in smaller volumes so they're expensive compared to a small pair of bookshelf speakers...... but they perform so much better.
B&W makes the DS6. The original version was designed for Dolby Pro-Logic, and rear channel Pro-Logic back in the mid 90's didn't need a full frequency range speaker. The original DS6 speakers were rolled off above 8kHz. That's not enough frequency extension for DD and DTS which have full frequency range surround channels. The DS6 was replaced with the DS6 s3. There were two major changes. The first cured the frequency range issue. The second answered the question raging at the time about monoploles for multichannel music versus dipoles for multichannel movies and which to choose for a system that was expected to perform well for both. B&W made the DS6 s3 a combined monopole/dipole speaker and fitted switches so the mode could be changed according to the source. Well, here we are 10 years after that debate was raging. How many multichannel music SACDs and DVDs do you own?
I'd always advocate dipole surround speakers unless your living room is so large that you can fit a single bed in the space between the sofa and where the surround speakers will be. That's the only way a monopole (bookshelf-type) speaker will have enough space to create a wide semi-diffuse surround field. Dipoles work better in the sort of space we have in the average British living room.
My two remaining reservations about the DS6 s3's are costs and looks. Firstly looks; I think they're brutal and that's not a compliment. Then there's the price, even secondhand. They're £600 new and not many people bought them, so used prices are high. Personally I'd substitute the B&W rear for the simpler and far more affordable Monitor Audio BRFX or BXFX surrounds. You won't hear a tonal difference, but the money you save will half pay for a decent subwoofer, and that is a beautiful thing.
So.... subs. One 'word'....
BK.
Pound for pound it's hard to find better performance for less. The XLS200 and the Monolith are the two products of choice if you can afford them. The entry-level Gemini II is great.
