The RU6, in the space of about two weeks, has already become a daily part of my music-listening life. It's just a damn good device. Waiting on my case to arrive from Cayin so I can protect it from all that mean it harm...
On the topic of the Sundara, it's a stellar planar but especially with amplification. Despite its low impedance and quoted high sensitivity I still prefer it with an amp. Listening to it on the RU6 it is clean, musical and a comfortable listen but just doesn't have the dynamism I know it's capable of. But each to their own and to have something to hand thats comfortably mobile and such high quality for such a reasonable price is great for everyone!
My current dream pairing is the SR80x > RU6 > phone. It's such a lightweight set up thats so versatile and the RU6 commands that 40mm driver without issue.