Truth is, it could be any number of things and the only way to find out is by changing one thing at a time. Zu's are widely regarded as being 'able' to lean towards a forward sound. One mans forward is another mans exciting so YMMV. I think Macca is likely on the money. Certainly my limited experience of them is that they aren't good for head banging.
Kuzma Stabi/S 12", (LP12-bastard) DC motor and optical tacho psu, Benz LP, Paradise (phonostage). MB-Pro, Brooklyn dac and psu, Bruno Putzeys balanced pre, mod86p dual mono amps, Yamaha NS1000m