I thought you'd biamp since I couldn't figure out what you would gain from using the preamp of one to drive the poweramp of the other. From what I remember the 8000S is a more capable amp than the 8000A, but the 8000A has tone controls and a phono preamp, which are both not present on the 8000S. Apart from the phono preamp I would think that both the pre and power stages of the 8000S are superior to the 8000A.
Unless you need the phono preamp (and I'm guessing you don't, since you don't have one on the 8000S) then I'd use the 8000S as an integrated, driving the high frequencies, and the 8000A as a power amp, driving the low frequencies. I don't know whether the power amp section of each amp has the same gain, but I think it is quite likely they do. You should get a noticeable improvement if you bi-amp this way, as you've almost doubled your power, and your current driving capability.
Please let us know how you get on ... it's an interesting experiment.
