Where to plug in RCA's

[Prowler573 5,000+ posts]

what LP crossover on the amp are you talkin about? the x-over freq. knob or the subsonic?

There is a LP (or Low-Pass) crossover built into that amp which filters all frequencies above the user-picked cutoff frequencies away from the speakers being driven by the amplifier.

my subsonic filter is on btw

Are you using a ported enclosure? If not the subsonic filter won't be especially necessary.

also whats the x-over freq for? it goes from 50-200hz, if its the x-over freq that goes from 50-200hz can u tell me how to set that lol i got no i dea how to set it, right now its at the second black mark after 50

As stated above the x-over in that amp is a low-pass crossover. It is to keep all frequencies above the desired range from being introduced to your subwoofer.
Common wisdom dictates that at 80Hz and above sub-bass starts to become localized (where you can easily discern where the bass is coming from rather than simply being enveloped by it) so you'll want a setting below that.

Start at like 75Hz, (the numbers on that adustment dial are frequencies measured in Hz....the 50 'mark' you mentioned is for 50Hz - meaning that frequencies at 50Hz and below are sent to your sub and everything above it is filtered out) slowly make adjustments until you can't tell specifically where the bass it coming from, and see where it sounds best to your ear. I can't make a concrete suggestion regarding where you should put it as it is too subjective of a thing....it's all about what sounds best to you.