KENWOOD!! :D People just don't say that name enough around here.
But seriously... signal to noise ratio is the... umm... ratio of signal to noise at the input to the internal amplifier. A higher number is better. If you're going to go with a head unit over $300, make sure it has a SNR of at least 100 dB. $200-$300 you should be looking for 95 dB and up.
Another number that's really important is the THD, the total harmonic distortion. I won't go into what it is here, but the lower the number the better. This number applies to the internal amps on the head unit, so if you get a really high end unit with no internal amps, it doesn't have a THD. Anything over 0.1% is unacceptable. 0.08% is good, and as good as it gets for most car audio stuff. 0.06% is great, but I don't think you'll find that on anything but really high end component amps.
One other thing, if you want to eventually hook up an amp, look at the preout voltage. Don't get anything under 4 volts.
:iagree:
My Kenwood head unit is really good sounding, but its great because of the features. It has digital filters on all the outputs... high pass, low pass, and adjustable Q (which kicks butt). It has level controls for each source (tuner, CD, satellite) as well as for the preouts. A 3-band EQ with adjustable center frequencies tops it all off. Being able to tune the sound to that degree really makes it a gread head unit.