I would avoid linking the high and low frequency outputs -- likely to result in smoke.
If your surmise is correct -- and if the garage wall speakers have low and high frequency drives, with a little re-wiring you could bypass the crossover in the garage speakers and wire direct to the drive units.
This isn't ideal because the electronic crossovers in the Sony will have been tweaked to optimise bass in the (doubtless small) speaker boxes supplied.
Or (worse) the Sony speakers may actually contain a power amp and the signal wires are just carrying a line level input.
Simple(?) solution is to use the Sony speakers and (if necessary) extend the wiring to reach.
fihart, I decided to just pack the unit back in the box, return it and bought a basic 200 watt, 2 channel receiver which works perfectly without having to worry about the possibility of "smoke".
Thanks for your reply