Well not for the 1st time I can answer my own question. I'll put it here for the benfit of anyone that comes along.
The pre on the LH500 includes EQ, limiter and volume before the FX out. Using FX out to the line level input on the Pro does work with the BassPOD output feeding back in to the LH power amp but the whole thing sounds better with the bass going into the BassPOD and just using the power amp on the LH via the FX loop return. The LH volume then has no effect and it's all down to the settings on the BassPOD.
Peter