Luckily there are some metal records published in HiRes (I've got some Gojira and others in HD tracks for trying) also even a lot of underground bands are taking care of production. The bad side is that the majority of my collection are in the raw and dirty sound side.
I love piano recordings and Chopin is always in my play list (bought some albums in HD tracks too). So if I get some improvement as you mentioned I will update to ZX100 because have found it at 349 Euros.
Moreover, I think that LDAC will help a little when wireless pay (z3 lacks of it)
Regarding the stalepie comment, but sound is not produced/sent by the DAP? Although later it goes through headphones' DAC (sorry if this sentence is senseless, but I am new in this worl)