Hello and welcome...
So first lets address the 7.1 vs 5.1, there is a list somewhere with the names of all the movies recorded in 7.1, which is pretty short, I just prefer 5.1, 7.1 seems to take you out of the action when the rears light up... Just preference, and with your budget I would think 5.1 would be the best bang for your buck...
Next "biamping" don't waste your time... Ill let someone else go into that in more detail if need be...
Next is brands... I would go Internet Direct, I have played with a lot of speakers and ID companies offer the best value{no middle man}, customer service {in most cases you can talk to the owner of the company}, and quality{most of the time hand built, or very close to it}...
Next is subwoofer------- How big is your room? My favorite all around sub {and I have 2 of them, but use 1 for the most part since my wife complains when they both are on in my medium large room 400sq ft} is the HSU VTF2
VTF-2 MK4 Subwoofer , so guys around here like rythmic, some like SVS I have heard them all and I like HSU and there prices.. Plus the nicest customer service you will ever deal with....
So if you get a single VTF2 that will leave you with around $1400 left for your 5 speakers...
-As far as towers vs bookshelfs, you are going to be crosing the bass out of your front 3 speakers and redirecting it to your subwoofer, this is common practice when useing a sub, your AVR will most likely shut your fronts and center down at 80hz and send that to your sub, and since a set of towers main benefit is a superior lower end, you kind of get all that bad things that come with a larger enclosure {resonation, space, cost, ect} with none of the benefits. SO you can get a much better bookshelf and Ill be honest with you, will put them Cambridge units to shame 5 times over...
I would get the Ascend Sierra 1's
Sierra-1 Bamboo Loudspeaker and sierra 1 center
Sierra-1 Reference Loudspeaker which they have rite now in Bstock in gloss espresso, the also have a set of 200se's in bstock, your total would be $1421 in Bstock... and that would be the gloss espresson sierra 1 front end, and the black 200se surrounds....
Ascend Direct
I don't think you will find a better sound for your budget.. That sub shakes my house which is over 4000 sq feet with ceilings as high as 22ft. And I own sierra 1s that I use for music, they are one of the best values in home audio, I use 340se's in my theater, which is also another option but if you can afford the Sierras and they are in Bstock {which is rare} go for it....
Anyway, good luck, I hope this makes sense to you, towers are fun, but you won't afford one that sounds as good as the sierras and you wont get anything extra since the low end is a moot point once your AVR crosses it out....
If you go with the advice you get from this forum, that cambridge 5.1 is going to sound like a McDonalds order window speaker in comparison....