How do I know if I can bridge only 2 channels of a 4 channel?
 

Ok, so If i wanna look at 4 channels and i wanna run my fronts[infinity perfects] off 2 channels and bridge the other two channels to a single sub (10w6S1) how do i know if a amp is capable of that? Some body said the JL 450/4 is capable cuz it has 2 power supplies. Is this necessary. WIll the amp say it can be bridged on only 2 channels? I want some thing a little cheaper than that 450/4 suggested if i could.

most of them are bridgeable. If you look on the amp and you see 2 channels with the word bridged under it and its two ohm stable, then yes its bridgeable. It would probably help if you listed the amp so the audio pros of the site can help you out. I've done it b4 where i bridge the last 2 channels... doesnt work well but maybe thats just me.

As has been said, most amps are bridgeable. Read the specs and it will tell you.

Cheaper amps you might look at:

MTX Thunder684
US Acoustics USX4085
Pheonix Gold Octane-R 8.0:4
PPI PCX-480
Hifonics ZX6400

OK, so if a 4 channel says bridgeable at all that means that you can bridge 2 channels into 1 always. I thought it might just mean that the amp was bridgeable like 4 in to 2.
Just to be clear, what I am looking for is 2 channels bridged and 2 channels not bridged.

Yep, just about all the amps I've seen are able to do that, I'm doing that now, so kind of in total, 3 channels. If you look at the spec of some amps, that say for example:

50 watts x 4 @ 4 ohms
100 watts x 4 @ 2 ohms
200 watts x 2 @ 4 ohms

typically you can bridge just two channels, and leave another pair in stereo.

you can think of all common 4 channel amplifiers as two independant two channel amps.

Really, I didn't know all 4 channel amps can be ran in 3 channel mode. Will the bridged channel, since it is an AB class amp, sound worse than a class D amp. I think class D mono amps mean they are just more efficient, no? I'm not sure, but this might make me buy a good 4 channel amp and bridge for the sub too.

Maybe Phoenix Gold's Xenon X200.4? It's cheaper than the JL 450.4 and is still a high end amp. So do you guys think this would be better than getting a X100.2 and X400.1?

no, the sound quality does not depend on whether it is a class a/b amplifier or a class D amp, or whether its bridged or stereo. What will matter is build quality of the amplifier, the design, and the components used. youll have to make a decision based on how much money you have versus how much power you want versus what amplifiers will perform to the standards you will be expecting, compared to the speakers you will be running (impedance, for example).

good luck with the hunt!