I don't see much of a problem but I would use the main outs with "Y" cables and it should be crossed over as low as possible to keep it from being sloppy.
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Yes, I would say that subjecting your tubes to vibration (through floor and air) is a bad idea. It's not so much that it will damage them, but it will screw-up your sound.
Use a longer cable to the sub and move it somewhere else. Make sure that the Fisher is isolated on a good damped stand, and if you do decide to use a sub near it full time, invest in a tube damper accessory like Herbie's Halos.