I would like to warn you guys on something here. I've tested the Onkyo and also FiiO X7. When Wi-Fi is on, there's a noticable background noise on the X7 and you can do nothing about it. Whenever the Wi-Fi is on, it doesn't matter whether you're streaming or just listening your files with the built in music app, there's noise. It's annoying to me.
And DP-X1. Well, there's also a hissing sound on that as well, when you streaming with Wi-Fİ. But I think maybe the problem on the Onkyo is somewhat different which is when you use another app instead of the stock music app, it starts hissing. Not sure which is the exact problem because I didn't have a chance to test for a long period, but the sure thing is those 2 DAP's have a background noise issue.
Never tested the Pioneer DAP, so can't say anything about that one.
ZX2 is still the best DAP for me in terms of battery life and having no problems like hissing on Wİ-Fi and so on... I agree it's beginning to get old and Android version is JB, but it still runs fairly smooth and with the unrivalled battery time, it's still the number 1 to me.