one foot long, each side short as possible
Ordinarily, a short(er) cable (anything under 5 feet is sufficient) will help preserve the limited damping capability of a tube amp. However with (especially) a braided cable, like the Kimber, a very short length may work against you because of the low frequency oscillations (some people call them 'reflections') that can form in a cable that has high inductance and capacitance like the braided Kimber. (This is common in undersized power cords also.) I would suggest you try a cable of 4-5 feet in length and preferably solid core (or very few strands.) If you want to do an inexpensive experiment, go to Home Depot or Ace etc., and get four 5 foot lengths of insulated copper house wiring, 10 AWG, and see if that doesn't improve your bass performance.
The (current model) MC275 develops 90w/ch and should have no trouble driving an 802 with its 90dB sensitivity (unless you have a large room and/or listen loud -- in which case you might want two MC275's in mono ;--) But it's important to note, that B&W specifies using very low resistance speaker cable with their speakers (probably for compatibility with their crossover insertion impedance), and that requirement translates into "fat copper conductor" which does not describe a braided cable (nothing against Kimber) but I know many people who love the sound of tubes with their B&W's, and they all use cables with lots of metal and very low resistance/foot specifications. Try it, it'll cost you maybe $10 ;--)
