Using solid copper Door Bell wire as speaker wire.

The last time I tried using door bell wire in my system, It sounded too edgy, and I got this terrible RINGING......
(You should have known someone would have gone there!)

Doorbell wire is 20awg, but thermostat wire is 18awg. Doorbell wire comes in pairs and thermostat wire comes in a variety of configurations including a pair, many odd numbered parings but also an 8 wire version.
If you took the 8-wire version, used every other wire to each of the two poles (4 wires per pole, pos and neg) then you would have an aggregate 12awg cable. The skin effect brigade will like the doorbell wire better. With that, I would use four of the twisted pairs, themselves twisted in a star-quad configuration so also 4 wires per pole, for an aggregate 14awg. Arguably, this would be the better cable of the two.
With either, use two separate runs for bi-wire. If you can find the wire with polyethylene insulation, that would be much better, IMO, than PE and even better than teflon. You could do much worse.

Double blind tests confirm not audible difference between coat hangers and Monster Cable. Just what we always suspected. ;-)

Cheap but crappy.