You'll need a switch to switch it in and out of the circuit, but yes, it's possible to use a tube preamp as a switchable tone/distortion box like this. You'll want to adjust the potentiometer for unity gain. If you're thinking of going in this direction, you don't really need a preamp, there are "tube buffers" with no potentiometer; look for one with unity gain so you don't have to adjust the volume when you switch it in and out.
The Schiit Saga has a unity gain tube section that you can switch in and out of the circuit with a touch of a button. Distortion is very low though so its effect on the sound is subtle and probably difficult to pick out in a blind test. Even higher distortion units may be hard to ABX... a couple examples were posted by members here before the holidays. If you really want a tube box as a kind of audible tone control, you might want to intentionally go for a high distortion model, like one of the Korg Nutube preamp/buffers. (You're likely to get a slightly higher noise floor though because the Nutube is hard to use without at least some gain, but it's likely to impart more of a DHT-like quality.)
