You should not connect two sources to a single input. Get a switch.
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I don't think your Quad has a build in Xover, so when you play two channel your sub would work with the full range signal, unless your Sony has a 4 outputs and a build in crossover. This is far from ideal, as the sub would overlap with the mains in the 60-120 frequency range. To do this right you need an external crossover.
The NHT X2 is an cheap active crossover that would meet all your needs - it has 2 channel and LFE inputs, allowing you to integrate stereo and surround sources with a single (or dual sub). Unfortunately they are no longer manufactured (I just sold mine).
Your other option is any other crossover and a switch.