EDIT: I just read your post back and I missunderstood... but rather than a boost, you just need a switchable volume box in the loop. really simple. Like a fixed 'volume' stomp box. Set up your amp for your lead tone, then stamp on the pedal to just drop the volume. It's like have switchable master volumes. If that makes sense. It would be easier than a volume pedal with a rocker unless you need the flexibility of the treadle.
A boost infront will change your tone a bit, you might like it. It might not necessarily boost volume depending on how your amp is set.
A boost in the loop... I dunno. If your preamp has a lot of gain you might risk overdriving the phase inverter which is my experience doesn't sound that great. but i might be wrong about that.
To keep the amp sound, I think the way to go is what I mentioned above, unless someone knows better.
