Everything has settled down now (headphone amp, cables) so I can offer an update on my experiences.
First, the headphone cable. I have a Toxic Silver Widow on order and will report in due course.
As it'll be a long wait I fancied trying something else in the meantime and got one from
jiangzhan2012 on Ebay; his 8 cores Hi-end 5N OCC silver plated one for Oppo PM-1. After running in, it is proving a very good cable and a useful improvement over the standard. The bass is tighter; there is more transparency and air; midrange may have a touch more clarity but it definitely has the same *fine* tonality (after running in! It was poor at first, be warned). Differences are not great but they don't need to be, and they are all positive, in the right direction (in my system). Sound is superb. So I give this cable a high recommendation ... exceptional at its price! Do try it ... just be warned not to judge it till it's had 100 hours use.
Next, comments on tonality.
I tried my AKG K812's and their U or V shaped frequency response was apparent - and the effect on tonality was clear. I could often hear that the fundamental and undertones of notes were audibly subdued compared to the higher harmonics. The result was that tonality sounded 'off' to me. Whereas the flat FR of the PMx2s keeps all these things in balance, giving a very correct harmonic structure and hence tonality. I don't tend to analyse things in this way, but I like it when I can link some technical aspects of a piece of kit to its sound (such as when I'm trying to get a DIY amp to sound good!).
Overall comments on the PMx2:
I'm loving them even more in every way. Gorgeous sound. And on going through my CD collection, I'm finding more and more musical gems that I didn't know were so good, enjoying them like mad.
The superb quality of the Gryphon Scorpio cd player is highly apparent; and a good tube amp works beautifully with them.
Music.
It would be wrong to turn this into a music thread but I feel that mentioning some more favourites, and how the PMx2s bring out the musical enjoyment, can be relevant.
Natasha Marsh, Amour. Pure *class* to her gorgeous voice and superb backing by the London Symphony Orchestra; Such delicacy, subtlety, expression, tone to her voice, spine tingly. OK, as much of it is in foreign languages it may not be to all tastes but do try it. The operatic style is not overdone. Ai Giochi Addio is superbly atmospheric; Les Filles de Cadix is lively, engaging, punchy.
Liner notes are excellent .. and lovely pictures!
Cathie Ryan, Somewhere along the road. Irish, with a pleasant accent (not too strong), Fine musicians, well recorded with great clarity. She sings beautifully with a pleasant vibrato (not overdone!), subtle touches to everything with expression, delicate variations and modulations; things like the h in 'where' and d's and t's of word endings can be alluring; tonality is superb. All these things are well captured, and relayed superbly by the PMx2s.
Sarah Jarosz, Undercurrent. If you like folk with a bluesy touch, try this; fabulous. Fine transparency to everything, tone to instruments, expression and emotion to her voice.
Applewood Road; the Sunday Times rates this highly but didn't mention the special recording quality; single mic analogue recording direct to 2 track tape, no editing or overdubs.
Can you tell I'm loving my headphone system to bits? And that's what it's all about.
Regards,
Mike
The tonal balance, low distortion, and responsiveness of the v2 PMx2's are a remarkable match to the Chord DAVE. As blown away as I was before, it feels like I'm really hearing what these cans are capable of for the first time. 
