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what I'm really saying is that I don't need much more power than what my stock head-unit puts out. Certainly that's not 50W/CH.


Yep. As stated typically around 15 true Watts per channel. A doubling of power will give you an additional 3dB. But to double perceived oudness you need 10x the power...
More available power will also let you stress the amp less [for better quality] for a given volume.

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I suspect that there's not much of a market for sub 50W stand-alone amps...


You'd be surprised... Of course most are advertised with far higher numbers But those advertised as sub50W would tend to the better [build&sound]quality end of the range.

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If I were a hardware geek I would look at stealing the amp board out of another head unit and packaging it up nice and neat under the dash.


Unless totally skint I wouldn't bother. Even a dead cheap separate lowpower amp often has better sound quality than the IC amp stage from a HU. After having bought my initial MkI shortly after a new VW Polo, cash was a bit tight and I went with 2 Pyramid amps off eBay. One 4channel (4x75W - yeah right) and one 2 channel that I ran the sub off. For the money, surprisingly good...

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Here's an audio-n00b question:
Do I really need a 4CH amp?
Isn't four channels the way that we get balance and fade capability?
That is, if I want to have balance and fade, must I use a 4CH?

You need 2 channels to separate left and right - otherwise you don't get stereo.
If you parallell your front and rear left (same for right) speakers and run off 2 channels you wouldn't be able to adjust the relative loudness of the two speakers (if different models it would depend on their respective sensitivity. You don't want one channel front and one back, as you wouldn't get stereo (left right)

Really low bass (sub bass) isn't locateable as higher frequencies are, so HUs with separate sub bass out often has just one such channel (instead of two for letf/right) Therefor it also really doesn't matter where the sub is placed. Which is nice as we can put the sub in the trunk where there's space, while speakers otherwise generally should be up front for a proper soundstage...

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I've read a few 'basic audio intro' articles, but I guess they weren't basic enough for me.
Thanks-


Don't know whether yu've seen this one, but I tend to always recommend this resource which starts with the true basics and goes on to pretty advanced stuff...
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/Michael