I know i need no less than 14gauge (12 probably better). I am going through walls and attic space. Left, right and subwoofer from a high current pair of amps. Suggestions? Nothing esoteric please, I'm cheap.
How well would this work (cut to three 65 ft lengths, one for each speaker and one for sub)?
I've no idea about individual code requirements, but (like you were proposing) make sure you go with a cable specifically labelled for "in-wall" installation - It will have the internal "pull cord" should you need to get some weight behind the install and the CLx or CMx designation allows you to feel safe about putting it behind your sheetrock.
I know that there are formulae for the conductivity of wires based on their cross-sectional areas, but in my (admittedly empirical) experience, the electrical differences between 14 & 16 gauge is really nominal, and the impact this has on sound quality and/or voltage drop is equally infinitesimal.
IIRC the difference in voltage drops (due to resistance) between 12 & 14 AWG is only a few hundredths of a volt for a 100' run, which you won't be able to detect without measuring equipment and definitely not by using your ears!
You might consider using a wire resistance calculator such as this one.
For example, 14AWG at 75 feet gives a resistance of 0.189 Ohms. Whether that is too much will be determined by the minimum speaker impedance. It's not uncommon to have 4 Ohm lulls in the speaker impedance . Assuming zero output impedance from you amplifier, the damping factor would be around 20 and in reality lower since the amplifier does have non-zero output impedance.
A damping factor of 20 may be on the low side. If it were me, I'd switch to 12AWG or even considering going to 10AWG romex for the run.