My thoughts on the N3 are as follows did all of my listening with a modded ATH ES 10.
Headphone Sound Quality
The HM 601 as the best for me, most natural timbre, excellent weight to the sound, most expansive imaging, great dynamics and macro detail. It glossed over some of the micro detail up top, the neat little background noises, though it did an amazing job with transients! Pretty good given the price, NOT on par with the Geek Out v2+ but for $125 pretty darn good!
The N3 on the Short Delay Filter was second best, brighter, more micro detail and less resolving of transients, amazing dynamics but it lacked impact and weight, often a little thin, but very fast, pulled out textures that the Hm601 often smooths, which can work against the N3, and other times works in it's favor
UCA 202's headphone out is terrible... very soft, REALLY deep but exceptionally narrow
Line Out Quality
The UCA 202 and HM 601 where excellent, the UCA 202 had a deeper wider more precise image overall but was still a little soft where as the 601 kept it's amazing timbre and weight with the sound stage opening up a little more, it lacked a little depth an precision compared to the UCA 202
N3 line out is a big improvement! Added body and better transient resolve, alongside better precision in imaging, but still thin sounding compared to the other 2 and not as expansive as either
Headphone Power Output
UCA 202 is appalling, hm601 isn't as impressive beside the N3, the N3 has 3 gain options and offers the widest range of power out options alongside having the most raw power on tap
User Interface
All HAIL ROCKBOX, seriously though I love it, it's faster than the N3 GUI and has more options, The N3's button configuration is easy enough to figure out, but I still find my self accidentally hitting back when I want to scroll down, though the two differ fundamentally. The N3 has a graphic user interface, where as rockbox is purely text based. Surely in today's day and age a GUI is more approachable and welcomed than a big list of text menus.
Function
N3 really surprised me here! The blutooth dac connectivity is a little noisy, but it's a good convenience. For what it's worth the N3 sounds better than the onBoard output of my LG V20... but I liked how many connectivity options the N3 offers, it's a Digital Jack of all Trades! The UCA 202 on the other hand is more of an analog jack of all, and the HM 601 has caveman level functionality... it play music or it not play music,
The N3 also offers a USB dac/amp function as well, allowing it to be pair'd with your computer as well as your cellular device, though I didn't get a chance to use it. As in my experience sound quality does not differ via the USB input vs playing files stand alone on a DAP
Battery Life
I charge my HM 601 like once every 2-3 weeks. An I get around 30 hours of playback time! The N3 I charged every 2 days and got maybe 8-9 hours tops...
Ultimately, for now the N3 and HM601 both have my recommendation,
more so the Cayin N3 seeing as finding a Rock Boxed hm601 is going to be difficult and it has next to zero support, where as with Cayin we get amazing customer service and a warranty. The HM 601 isn't really a practical recommendation, though pair'd with Rockbox it's a tough combo to beat for just a super basic digital audio player, but I was really excited to have had a chance to listen to the N3! I love the digital filters, and all the awesome functionality it offers, overall it's a better for portable music than the simpler hm601. Cayin took extra effort to integrate with and improve the existing cellular options, as opposed to standing against them as the HM601 and devices from that time often do
