I have a set of active monitors and 3 stereo input sources (audio interface, cassette deck, and a turntable). Would there be any problem (signal quality or otherwise) with building a passive stereo mixer with A10k log dual gang potentiometers for each input? Or would I need to build an active mixer in order to allow for some protection at the inputs?
Maybe 100k would be better
I guess you’d be OK if the devices are all line level outputs (obviously with a phono preamp for the turntable). If I were in your shoes and planning on DIYing something I’d probably try and make it active, just to satisfy myself.
That being said, I’d more likely grab one of these AU-401 mixers, they’re cheap enough and work well - I just checked and found I’ve bought four of them, three bare boards and one cased one like this:
Just make sure you provide a centre-positive and not a centre-negative supply if you get one, because having bought four I only have three that work now 
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You would need a resistor between each wiper and the downstream device to avoid, for instance, shorting one of the inputs to ground if its pot is at maximum and another input’s is at minimum. To keep the pots’ behavior correct those resistors should be large compared to the pot, or at least not small. So with 10k pots use at least 10k resistors, 100k probably would be better.
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