I've tried the Orphy, OTL, FTS, Yatono 8w and Shiro 8w with the Rn6. The Rn6 actually pairs well with a broad range of cables (unlike the Xe6 which is more finicky in that regard). But the Shiro 8w in particular is by far the best pairing I've tried. It of course makes a big improvement in the technicalities, like all of the aforementioned cables. But in addition it makes the sub-bass more impactful and articulate; it leans out the mid-bass a little bit; and it's just PERFECT for the treble.. you get bite, crunch and detail with no excessive brightness/harshness/energy. I don't even know how to describe it, it's like there's a bit of thickness to the treble -- it's detailed but not thin, nor is it dark.
Put it to you this way -- I opted to sell the Fei Wan in favor of keeping the Rn6, even though the general perception on head-fi seems to be that the FW is in a higher category of performance.
The FW does have better staging, but when amped (I use Tur08 or TsuSC), and with the Shiro 8w, the Rn6's technicals are about as good in every other respect (the cable & amp also help give the Rn6 more stage depth). Then you add the visceral impact of the kinetic bass, and the fantastic treble and tuning... I'd say with the Shiro8w and an amp, it's still in the summit-fi category, even today.