What are you trying to do? How many rca cables does your HU have? Some have 3, one for the front, rear, and sub. Why do you need a y adapter if you already have a sub rca on your HU?
i have already 3 but someone told me if i connect both subwoofer output on the head unit (left and right), combine them with a Y adapter and wire the amp just on the left RCA the signal is better. One RCA wire and not 2. What u think?
Nah, don't do that. If you already have 3 rca preouts on the HU, use it. The Y adapter is for jbl amp. The HU's with sub out, use that. If not, you can use the rear outs for the sub amp.