If cable possess high capacitance, the shorter the better. Don’t place the amps directly on the floor, of course; physical vibrations transferred into the floor will travel right into the amp’s chassis (even if it’s a cement slab foundation, though they will be less severe than with a suspended wooden one), especially bad with tubes.
Isolating the amps from floor-borne vibrations is always a good idea. If you’ve been following discussions on that topic here, you may be aware of roller blocks, springs, and the great Townshend Audio Seismic Platforms and Pods. The makers of all the isolation products tout their products effectiveness, but users sometimes disagree ;-).