Not an upgrade, but some times ago I bought a pair of big, heavy Quadral Argent 80 loudspeakers. As the seller just moved in his new house I couldn't listen. He swore they were good and as he seemed like a nice guy I glanced over the speakers and thought they were okay. Hauled them home and up two flights of stairs (some 30 kgs each) and connected them. Hmm... an odd sound from one of the tweeters. After a closer look one of the metal dome tweeters was replaced with a cheap piezo horn that was glued in the original faceplate and then covered up with the original metal dome. Of course the seller didn't knew anything and refused to refund. Eventually I tracked down a pair of original tweeters and restored them to their former glory. But that was the last time I trusted a guy on his big blue eyes...