What do LM78XX type regulators do with low input voltage?
Most likely they will be 'solidly' on and not regulating whatsoever. The internal feedback will detect that the output is too low and essentially saturate the series-pass element (as much as it can) meaning the output voltage will be less than the input, and unregulated. 

If you're set on trying to run with lower voltage, I suggest connecting the regulated adapters to the output side of the 78XX regulators, not the input side. That way you won't have that extra series-pass loss.

I imagine it should be fairly easy to obtain a small 60Hz transformer and do your own unregulated +/- 22VDC supply to feed those linear regulators, instead of using wall adapters.