Since you have the speakers I'd set it up in surround....you can always just use stereo/2ch mode too.
If this is a used avr first use the manual and do a full reset (also known as microprocessor reset/factory reset). This will put all settings at original default values and you can setup afterwards for your specific setup levels/delays/eq either manually or do it via Audyssey (did you get the Audyssey mic with the avr?).
I'd use the XLS215s as front left and right, the Klipsch as center, the Infinity 363s as surrounds, the T15 as rear surrounds (if you have an appropriate room for 7ch setup....what size room and where is your seating in the room?).
The Niles amps aren't going to make a huge difference from the avr's but to use them and take a load off the avr's power supply it might help out a bit. To put both Niles amps to use, I'd probably use one for the L/R and one for the center channel (just one channel of one amp for the center, I wouldn't bother with bridging). The avr alone without the Niles amps would probably be just fine, tho. You do have limits, tho....if you want a foolproof system no matter how much you twist the volume up, meh.
Do you need help with appropriate interconnects for the avr to amps, if you go that route, or the speaker cables? Simple relatively short rca cables from avr to amps are all that are necessary and are readily available from a variety of sources for little money. Speaker wire need not be complicated....try this site for pretty much anything related to speaker cable
http://roger-russell.com/wire/wire.htm to start.
I'd say the Cambridge is just extra baggage so far....the dac in the avr is quite fine. What you need are some subs, while the CVs have some decent bass extension, they're not subwoofers.
