A preamp should be able to do both.
whether you choose to use it to just change the eq, and set the output to match bypassed signal is up to you. I’ve got a Dave Hall valve unit that I used to use as a preamp direct into a power amp - much like other BCers who run into the effects return socket of their amp; these days I just use it as an overdrive pedal, so I dial back the volume and that’s what it is.
Tone shaping is much easier (and safer) to do in the preamp stage, as the signal is significantly smaller (typically less than 1v) when compared to the signal coming out of the power amp stage, so that’s why it gets done at that particular part of the amp.