I'm no expert and home theater isn't my focus, but I've really gotten into surround recording of music and have played around alot with playback for editing and my own enjooyment. I'm using an inexpensive Panasonic class-D HT reviever that sounds way better than it has any right to. Similar to your Cambridge in function: 7x100wpc which auto-bridges amps for stereo or bi-amps if you biwire the mains using both the A and B terminals. I figured I would have upgraded it to something superior by now but it's good enough that I haven't bothered yet, even driving multiple juice hungry speakers.
I'd start by using any decent old speakers you may have on the sides and rear. When a better pair arrives, I tend rotate the older ones towards the back of the room, keeping them in symetrical pairs. Figure out where the placement works best (just as important as with the main stereo speaker pair or as with mics- optimizing location ,location, location is more important than most everything else). Do the level and distance set-up thing on the receiver and balance the tone as best you can to match them to the fronts if there are L-pads on the speakers back there. The distance/time adjustment is critical to good results IMO, the level you can adjust by ear with pinknoise, or pull out the sound level meter, but it's easy enough to hear when its too loud or soft. When you figure out the side & back speaker locations that work best within the compromises to be made you'll have a better idea of what kind/size/shape of dedicated speakers to look for.
As for what speakers to buy at that point I'm not much help. I will say something on the dipole vs bipole vs monopole thing: I bought some second-hand B&W surround speakers for two reasons:1) they were a bit more likey to tonally match my B&W mains and included a tweeter level adjustment switches 2) they are dipole / monopole switchable so I could play around with that. I found their tone was the most important thing to match and that the dipole setting always sounded less transparent & slightly rolled off in the treble, regardless of the treble boost setting. I thought I would prefer the dipole surround setting, since for me, the key aspect of surround (music or cinema) is just that- the surrounding sense of immersion throughout the room. But I keep them in monopole primarily to keep the treble balance even throughout the room.
The problem then is that as monopoles, the surround speakers become much more easily identifiable as widley spaced, point source reproducers of sound. They sound like a couple speakers sitting somewhere back there to the sides. To compensate and 'fill the big gaps', I've sort of gone the big theater route by using multiple, paralleled speakers on each side to illuminate the room more evenly and diffuse the side surround a bit. So the 2 B&W surrounds are wired in parallel with some Klipsh bookshelves and the Panny has no problem with the 3-4 ohm load. Im only driving one center rear speaker, partially because the room is setup on a diagonal, partially because I'm using the other as my front center identical to my mains.
So I end up with 8 speakers around the room (not counting the ones which aren't hooked up, sitting in corners or acting as end tables
) driven by 6 amp channels: three matched floor-standers across the front, the Klipsh pair directly to the sides at 90 degrees, the B&W surround speakers at about 135 degrees an a single B&W802M in back at 180degrees which matches the three in front. Yes it's hodgepodge, but it works really well and importantly to my ears, you can't easily identify the sound as coming directly from any of the surround speakers, it just envelops the listeners, even if they sit sideways, just like at an actual music event or movie theater. Yet there is plenty of directionality for movie wiz bang effects in specific locations. It sounds right.
Way to many words there. To sum up my experience I'll just say that speaker location is the key thing, try to get them far enough away and evenly arranged around the space. Getting an even tonal balance is second, and having surround speakers that can reach low enough is third- the real sense of envelopment and one of the biggest benefits of surround comes from the great dimentional aspect which multi-channel reproduction imparts to the bass (not the lowest octave sub rumble stuff, I don't have a sub either, but the real bass a few octaves above that).
It's cool hearing the crowd applause erupt all around the room. Email DPA for their excellently recorded free surround sampler, find that Alan Parsons DSOTM quad mix and give it a listen, rediscover the music all over again.