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Output levels vary depending on the manufacturer of the audio component in question. One thing that is nearly universal is that "line-level" outputs are low impedance (typically less than 200 Ohms or so) and line-level inputs are almost always >10K ohms.
Since you are building the preamp/switcher yourself, the best thing to do is measure the output levels of the components in question. CD players are easiest, because you can pop in a test CD (and you can make that yourself) with a 60 Hz, 0dB sine wave and read the maximum output with any multimeter.
Tuners, TVs, etc. may not be so easy but it isn't going to be critical anyway. Most multimeters aren't any good at measuring AC voltages that aren't sine waves, so your best bet is to use an oscilloscope. If you don't have a scope, then just assume the output levels are about the same as the CD player and leave it at that. You can always change amplifier gain later by swapping out a feedback resistor or padding the input.
If you want to get fancy, you can use a uC to make an adjustable attenuator and switch inputs, and make it programmable so you can match the levels depending on which input is selected. That sort of thing takes some pretty simple software.
I_F
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need list of "standard" values such as impedance
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