#8 won’t fit any receptacle. There is NO need for anything beyond #12 on a dedicated 20 amp. If the run is so long there is a voltage drop, you should have ran a 220/240 sub panel anyway. Save your money and BUY good copper Romex along with Hubbell receptacles.
Use L1 or L2 but don’t use both for the dedicated circuits. I don’t know what your wattage use is but #10 will run a huge AC unit.. Are you trying to make sound for a football stadium?
I use SOOW 4/3/g for an extension cable in the back yard, that runs anything including my stick or wire-feed welders. The wire-feed recommends #10. What are you running? :-) With that SPL potential you could use it as a personal home defense system. Brain scrambling able.. Kill every bug within a 5 mile radius. LOL