If you're going into the input jack of any guitar amp, you've got to remember that the Treble-Bass-Mid controls will affect the sound of the pre-amp. That maybe be a good thing to you or it may be a bad thing.
On the good side, you've got an extra set of tone controls. You can give it some extra scoop. If you were use the classic Treble=6, Bass=6, Mid=6 , the traditional Fender tone stack will give you a wicked deep mid scoop around 500 Hz. Some people like that sound. For others, that is too much of a change. It may color the sound of your pre-amp too much.
If you want to avoid this, you should use the amp's "power amp in" jack. That'll bypass all of the controls on the guitar amp. If it doesn't have a "power amp in" jack, you should plug your pre-amp into the "effects return". If it doesn't have one of those, then you're forced to plug into the input jack.
Going into the front of the amp like this, you're forced to go through the tone controls. To minimize their effect on the pre-amp's sound, turn the Treble and Bass down to zero and turn the Mid up to 6-7 (mid doesn't matter too much when the T and B are at zero). Contrary to what the numbers on the knobs say, on a traditional-style Fender amp this T=0 / B=0 / M=6 configuration will make the EQ flat. It'll have no high, mid, nor low emphasis. It'll just be even. That's probably the sound you want for your pre-amp.
Chip