The stock infinity hu doesn't have preouts. The hu main-amp outs are wired to the factory amp. In other words, this is high-out to amp wiring. So if you want to use the stock infinity amp it doesn't matter if your new hu has 2 volt, or 4 volt pre-outs.

If you buy a new hu and a new amp i would strongly recommend an after-market hu with 4 volt pre-outs and an amp that can handle that,

Regards
Markus