Voltage drop allowances are not even a NEC requirement, I know I've had voltage drop with my system when I used a carver subwoofer for a short time, even with 20 amp dedicated circuits using 10awg wire.
Dedicated circuits
I just completed installing 2 dedicated circuits. After reading several threads here, I went with 30 amp breakers with 10 AWG wire with high end receptacles. One circuit for the amp and the other for everything else. I’m blown away by the difference. Tighter bass, not as bright, better imaging and soundstage. Should have done this long ago.