I have a Chord Chrysalis DIN to RCA interconnect, where the DIN plug is specially wired to take the tape output from the NAC 82 (the Chrysalis is believed, around these parts, to be the most synergistic with Naim, but of course, it's not actually Naim it's going to, it's Stax!).
I have some other cables I've tried: the Chord Cobra is more extended, but seems to have less PRaT, a Preh DIN/Van Damme cable/Neutrik RCA combination does a good job, but is not vastly different to the Chrysalis.
I've even tried a special Chord Chrysalis 4-pin DIN to RCA interconnect, running straight from the Hi-Cap (which powers the Naim Prefix head-amp and which supplies the signal to the pre-amp) to the 717, thus cutting the pre-amp entirely out of the circuit, and this had a similar effect to the Cobra: more hi-fi, less music.
I'll have to take a look at getting a Naim pre-amp output straight into the 717 (after having altered the internal switch!), and I can also investigate using balanced interconnects instead of the RCAs, and I still fear that your first remark may be salient here: "Naim gear is far from neutral so that's your problem right there."
This would also explain why I've had such a good time with the SRD boxes (and you may have noted from my Head-Fi posts about this preference): it's when I'm using these with Linn or Naim amplification that I get what I'm looking for with Stax headphones.