Im really new to amping. I have read alot of people say you never want to run your amp at full volume? But why is that exactly? I like to crank my amp all the way up and use my phones volume control instead i just prefer it that way but why is this not the best practice?

I am looking for a good cheap entry-level DAP i want to go as cheap as possible. I currently looking at the xduoo X2.

How long should I expect my KZ headphones to last (EDR2,ATE S,ATR) with moderate use I also sleep in them? I'm not rough on them