As far as I understand the reference guide correctly, it should be possible to set the LR-send to the matrix as pre-fader, which should give you a significant increase in signal. If that’s not enough, you could also try setting the send to the matrix to >0dB to introduce some gain beforehand.
The Matrix also features a full set of processing which should allow you to use the compressor’s make-up gain for further improvements while limiting the output just below 0dB.