Hey Jan, something is not right there if the supreme "did not sound very good". If you are using it as a bypass cap, then no amount of burn in will change the lack of chemistry with the cap you are bypassing. If the cap is on its own, in a coupling position or other signal path critical spot, then, I would say that the problem may be somewhere else and the supreme is making it worse.
From my experience, the Mundorf supreme 'house sound' is very precise and reveling, with good timber and depth, but there is a certain 'window' for the highs and high mids to make their presence know. They get out of the way of the sound if you will. I have used them as bypass caps, coupling caps, crossovers etc. I love em. Just wish they were cheaper!