Mostly one would use a Pre-amp to preamplify a low level signal up to a standard or high level.

This signal could then be fed into line level devices which may include a Compressor.

If the compressor is a Foot Pedal (stomp box) compression for guitar then you would use it before the pre-amp.

I supose if you were to use the pre-amp as a distortion you might feed the compressor into the pre-amp.

If it sounds good ... do it. :w:

I don't know the Manley in detail so I shouldn't comment.

Take a different box the LA2a.
It has so much level available in it at both the input and the make-up that it could almost be viewed as a pre-amp.

It is not a pre-amp though and in most cases the input is heavily attenuated then in hits the opto and then through the make-up. As I said the make-up has so much gain it could look like a pre-amp.

If it sounds good .. do it!

Most compressors won't get enough gain from a mic level signal to work effectively. Usually you will need a preamp first to get the signal to line level. It is true that the VoxBox lets you put the compressor first in the chain, but I don't own one, and I'm not sure how they make that work.

The pre-amp is probably first up and then you can chose what is next, like comp or EQ .. as I said I don't have a Manley so I haven't flipped the lid to find out.