haven't read the whole thread, but has anyone suggested xylophones? Is there a "bass xylophone"? Wooden xylophones will tend to sound a bit "darker" than metal ones.

If you can break your self-imposed rule about electricity, then treat yourself to a Stylophone. They sound great when you tune it as low as it goes and turn the volume right up.

In fact, get a Stylophone anyway, irrespective of whether you use it as a bass substitute, they're so cheap on amazon...