when i turn on my sub i can barely hear any sound. the amp is on and i can tell that the sub is on but the volume is very low. even when i turn up the volume on the headunit i cant even tell the sub is on. any advice?

alpinr 1241d type-r 12"
jbl px600.2 amp
alpine cda-9805

i rechecked the rca connections, made sure the ground was secure, tested the voltage with a multimeter to the amp and its fine, fuses are fine, turned up the sub level on the hu, not wired at 8 ohms, changed the gain, rechecked the polarity several times and its all correct, set the filters to every setting.

i have no other amps or subs to test with.

any other advice?