One thing to consider is that you still get a separate zone with the C15 or C10 and, while the SQ is not as good as the other options discussed it is still good. And the earlier C10s did not upsample, whilst the later do but do not have apodising like the C15; so it might make sense to stick with the C15 for a secondary zone. I use mine to drive DSP3200s in a place on my veranda for parties, etc.

MD600(2x2TB); iPad Pro
LR. 818v3, 8000SE
HT. 861v8+ID41,HD621; BW 2x802D,HTM1D,4x804S;SVS sub; Mac 5 MC501s; RGP 600Pro, JVC X950; Radiance Pro; Oppo 203
FR. G65, DSP3200-3300 DSW, HD621; Mac MC152; Oppo 103, PDP TCP65VT60, Radiance 2124; RGP1200C
4. G65, DSP3200-3300,HD621; OPPO 83, Radiance XE PDP
5. 818v3; Prime HP Amp; HD800