No experience with the Zu, but another upgrade option for your Heresy I speakers is to upgrade or replace the crossovers and tweeter with replacements from Bob Crites. I did this to mine recently and am very happy with the results. The parts in your crossovers are probably 30 or 40 years old. Mine were all dried out. The tweeters Bob offers have more extended range and add more detail without being "bright" or harsh. Cheaper than buying new speakers, relatively easy to do, and a nice sonic improvement.