First off, I respect Tom and his work a lot and he is a great analog engineer, but not a magician. As someone with an engineering degree I don't think there is any need for this terminology in this context. There is engineers, good engineers, great engineers, but no magicians in my book. Magic usually can't be explained, while engineering is the exact opposite of that. We mathematically model (almost) everything we can, as precise as feasible/practical and then try to optimise what we can, given the technical and financial constraints. If being good at that makes someone a magician in your eyes, so be it.
Secondly, I think I am aware of most advantages and disadvantages of higher supply rails. I didn't check the Phonitor thing in detail, but I remember it said Headphone/PreAmp and I didn't find a serious reason why one would need such high rails (which doesn't mean there isn't one - I might have overlooked something). I am not a studio guy, but if I remember correctly the peak line level in professional environments is +24dBu, or around 12Vrms (*sqrt(2)*2= 34V p-p). There are off the shelf OpAmps that can handle this. But sure, if you want to drive 1W into 600Ohm Headphone loads, you won't get away with that. Then again who needs that, even in a studio environment? I don't see how cascading gain or "a lot more gear in the chain" creates a need for such high supply rails. Or what this has to do with laid-back-listening?! There are certainly good reasons for higher supply rails, but I struggle to identify them in a Headphone Amp. Be it for consumer or professional environments.
Last but not least: Maybe Tom will do a DAC in the future (although I don't necessarily expect it), but why should he use the AK4490? There is a newer version of that called the AK4493 if I am not mistaken. If you mean their high-end parts its AK4497EQ and the new one is the AK4499EQ. I would also be interested to see what he would do with it, but I suspect it wouldn't be much different from what other designers do. No point in reinventing the wheel. I personally would stick pretty close to what AKM themselves do for their demonstration circuits. They use lots of OPA1612s for the I/V and differential stages, I think at ESS they do the same thing on their evaluation boards, so does TI in application circuits for the OPA1612.