Hi Kev,
I've done exactly that (except that I have a source selector switch in the chain too). If you have only one source:
CDP->Input RCA->Volume Pot-->TLP Input -- TLP Output-->Power Amp
.......................................
.......\-->4k7 resistor-->Sub XO--> Low Pass goes to the Sub Amp
(ignore the ....dots.... they are there simply to get the spacing right)
To (hopefully) make the diagram more clear, from the top of the volume pot you will run two sets of wires:
1. The normal signal and earth leads to the TLP input
2. The second signal lead runs through a good 4k7 (or similar) resistor (to provide some isolation between the sub XO and the TLP... this is a step to reduce noise injection into the TLP), and from the resistor to the hot side of an output RCA that will be connected to the sub XO. You will also run a wire from Volume Pot Earth to the earth of the output RCA (that will be connected to the sub XO).
I built it this way because of suggestions (long ago, maybe even on the old Harmonic Discord board) that the sub out should be taken before the tube stage.
Until a month ago, this setup was in my main system (now I'm listening to my GK-1R

and the TLP has gone to the bedroom system); this setup can be built to be very quiet.
If you have more questions, I can send you pictures of my TLP layout.
Good luck, have fun.
Peter