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[da_bull]

2x20 amps, D3ity is right just add 'em up, use a 40 amp main fuse. 8ga. wire is fine for that amp. 8ga. is good up to about 50-60 amps in a typical 18 foot car run. If you have more than one amp you will have to add that one's fuses in too. Or run another wire for the second amp.

Switches on your amp:

1. This is the RCA selector.
[1(mono)] is if you are using it for a sub amp only (or kind of a center channel) and only have 1 RCA, or in the case of your 4 channel, you can use it as a larger 2 channel.
[ST] is a typical stereo input which is what you want for your setup.
[1+2] combines the L and R RCA signal into one signal. This would be more for a center channel where you would want both left and right information coming from both channels.

2. This is the crossover selector.
[Off] lets the signal play full range
[HP] High Pass filter. It allows only the mids and highs come through, filtering out the bass frequencies (this is what you want on both channels)
[LP] Low Pass filter. Just the opposite, blocks mids and highs, just letting the bass come through.

3. Filter Frequency: adjusts how much filtering takes place. Set both of these at around 120-140hz at first. You can fine tune it later. Remember that the lower you set this, the better it will sound, but its a trade off, you cant turn it up as loud in this setting. I wouldnt go below 100hz. You can turn it up more by adjusting more bass out of it.

4. Gain: This determines how much of an RCA signal gets in the amp. A lot of people think this is a volume, but its not. Yes, it gets louder as you turn this up at any given volume at the headunit, but you can accomplish the same thing by turning the gain down and turning the headunit up. This way actually sounds better, with less static noise in the system, by letting the amp do its job of amplifying the given signal.

Lather, Rinse, and Repeat for channels 3/4. If you have bigger or better speakers in the front you can set the HP crossover frequency a little lower on the front than in the rear (i.e. 100 for the front and 150 for the rear, you can play with that a little to fine tune, to see what sounds good).

Hope that helps,
Bill...