that's gonna be tough with an mbox.
the problem is being able to monitor what you're doing, as the only real outputs on the mbox are the monitor outs; unless i'm mistaken, the s/pdif outs just mirror the monitor outs.
the way to work around this is to plug the monitor outs into the compressor inputs, and straight from the compressor outputs to your monitors. You may need to buy some converters for this, but it's all balanced, line-level connectors the whole way through (provided the compressor has balanced i/o which it should do).
Once the compression is set to how you want it, you'd then have to plug the compressor back in to the line inputs (for impedance) inputs of the mbox and record it in to PT, although you would have to set the outputs of the track so that they're not your monitor outputs otherwise you'd get a feedback loop. set it to be a bus or something - PT requires you to have both i/o assign in order to record a track i believe.
the only caveat is to check that you're not clipping the monitors by sending them something that's too hot, you won't have any meters between the comp and the monitors whilst you're auditioning.
unless someone else can think of an alternative?
