OK, I'm back.
Coldsnow, correct me if I'm wrong, but it sounds like what you want to do is to connect a simple passive Bass-Treble control between a line-level source and a mic preamp. The circuit you want to use was mistakenly referred to as a "Baxandall", which threw the whole matter into confusion because now everybody thinks you're talking about a specific
active EQ circuit that reverses signal polarity. But I'm pretty sure this is the circuit you're talking about:
I adjusted the values to give curves more usable for full-range music. The values you usually see in guitar amps give more of an upper-midrange boost which is great for guitar but not so good for program material. Here are the curves:
This passive circuit does
not invert the signal polarity, so you don't have to worry about that. It will cause phase shifts at the frequencies of equalization, but all practical equalizers do that.
We have to scale the component values to fit the lower impedances involved with a mic preamp input circuit. The mic preamp will typically work best from a source impedance of 150 to 200 ohms with a mean signal level of -40 to -20dBU. The EQ should work into a load of about 1K and its nominal attenuation is only about 22dB. Also it would be a good idea to isolate the EQ from the reactive elements of the mic preamp input transformer. So we add a pad at the output of the EQ, which adds about 15dB more loss and reflects a load of about 1.2K to the EQ, and about 200 ohms to the mic preamp.
It's very important that the EQ is driven from a low source impedance (as noted on the schematic); otherwise, the curves will become very screwy and the "flat" (midpoint) position will be anything but!
If you want a slightly more sophisticated passive EQ that can be inserted between a line level source and a mic preamp, let me know. I designed one a while ago.