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* What frequency range is generally considered baseline "good"?
20hz - 20khz is fine.
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* What S/N ratio is generally considered baseline "good"?
Most SNR numbers are something like 70-90dba, but in my experience, those types of numbers are easy to fake. A lot of it depends on how clean your local power is, and how good the stage is at rejecting power supply noise. I'd suggest not taking too much stock in this number and measuring it yourself.
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* Does gain generally introduce distortion?
Phono stages inherently have a very great amount of gain (100-10 000). Increased gain will improve some numbers and compromise others, depending on the circuit type. Noise generally goes up with higher gain.
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* Should I use a mixer or a line level preamp? Is there any advantage to using one over the other?
I would suspect there's no real difference; the preamp might give lower noise. A standalong passive preamp might be the best bet. Again, this is something that will probably require trial and error.
If you know the exact attenuation you need, you could probably solder together an inline attenuator that will definitely be the cheapest option ($10 in parts) and will have the lowest noise.
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* Are the preamps @ phonopreamps.com any good?
No. The TC-750 I bought had a ridiculously bad wall-wart power supply driving an all-discrete stage with virtually no power supply rejection. The actual phono stage seems ok, but you're going to want to shell out more money on a power supply mod if you buy from them.