Extension tubes contain no glass, they are just tubes, so they shouldn't cause any softening.
true, there are no additional optical elements, but it essentially it causes your lens to focus closer than it was designed to, therefore any lens issues there may be are magnified! It's probably trivial while using good lenses.
Other nifty thing to try would be to reverse your lens on the camera. You can find cheap adapter rings for that reason. I believe this is a better method as you do not use extra glass nor you magnify lens issues. eg:
Or even add a reversed lens on top of another lens (something I haven't tried yet):