If I remember the pre-release gossip on the TP, they solved this problem by putting jumpers inside the case which can be set -6 or -12dB to adjust the player to work directly with amps without fear of speaker damage. Maybe Modwright removes these as part of the output stage upgrade, but they might still be there. Maybe check the stock TP online owner's manual.
EDIT:http://www.slimdevices.com/documentation/Transporter-Owners-Guide.pdf
See page 24. It allows -10, 20, 30dB, and 0, of course. You may not need an attenuator if these are still in place.
Rich