Hi Morgan,
I had a pair of SongTowers with the standard H. tweeter (and not the older ribbon) mated with a Melody I2A3.
Agree with the other posts.
To add, I did generally find the SongTowers in my setup and room to be very smooth. However, there was brightness with certain recordings in the higher frequencies (which I did not attribute to a poor recording issue).
I loved the Melody I2A3, but as the review (6moons, if I remember correctly) points out, 18wpc will only get you so far with the lower frequencies. This is likely the more probable reason you are not getting as much from your electronica and rock recordings. Even with the Zu's high sensitivity. I have a pair of Zu Superflys and additional power does add to the overall performance.
Therefore, you may want to consider a changeup in your integrated as well.
However, understand that the SongTowers are Salks entry level offering, so moving up the line will get you more from / in that area, as well as gains overall.
In my opinion, Salks do better with more power than what the Melody delivers. If you are dead set against replacing the Melody, another option would be to get bi-wire posts, keep the Melody, and biamp with a (used and bargain) ss amp for the bottom frequencies. I personally never ran the Melody this way, so you might want to check with Hugh and determine if the pass through is good enough for you to keep the timing and coherence intact.
Good luck with your decision. Jim and crew make fine products. If you can swing it, I would move up the line, as it seems you are actively considering.
Also, photos will help. Can't wait to see your view of SF.
- David.