Raybo -- you are going to be pushing this to the limits
it will probably be a process of discovery but this is how I think it will work:
First off, in order to do 11-ch mode you will need a 2-ch amp to power the "wide" speakers. Since you have 13 speakers, it would probably be best to also add a 2-ch amp for the Front L/R also. So you would want a 4- or 5-ch external amp to power the front L/R and wide L/R (and the center also if you get a 5-ch amp).
9 Amps with 3 (I believe) assignable
actually, from what I can tell they are ALL assignable! you can put the receiver in "Free Assign" mode (check out pg 40 of the manual). In combination with the external amp above (powering FL/FR, WL/WR, and possibly C), you can then reassign the 8 internal amps to power height, surround A+B, and surround back speakers. The 4810CI doesn't use the "Surround A+B" configuration any more, but I think you can use "free assign" to accomplish this by assigning four amps to power the SURR L/R channels.
the 5308CI also had a "free assign" mode so hopefully owner can chime in, but from what I can tell from the manual this is how it would work:
- FL/FR, WL/WR (and C optionally) pre-outs to external amp for front 4 (or 5) speakers
- "Free Assign" the internal FL/FR amps to power SURR L/R (for Surr. A)
- Leave the SURR L/R amps as is (for Surr B)
- Use the S.BACK L/R amps as is (for Surr. Back)
- Use the Height/Wide amps to power HEIGHT L/R