would lowering the action of a bass that has really high action affect its tone?
Only ever so slightly. The only reason why the action would be lowered is to ease play and comfort. Of course, lowering the action does bring the strings closer to the pickups, and thus has some sort of change to the tone, yet I would see it (and probably hear it) as very minor, if at all audible.
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Actually, if it is a fairly expensive Bass then yes. If you get the action low enough so that it only starts to Buzz, you can get a really neat trebly fretless sound. With a LOT of trebley attack. It kills your sustain though. And it has to be set up perfectly.

Though if you don't want THAT drastic of a change in tone, just simply raise or lower your pickups. The farther they are from the string the softer they get. The closer, the more attack you get.