Since the main effect of the mod is the reduction of inner reflections, the reduced treble intensity is the result of reduced parasitic transient corruption – the direct sound from the membrane has the same treble content as before. Hence it is the better solution than a cable with the same treble-reducing effect, which reduces the whole signal, direct sound and parasitic reflections. That's not to say that all a cable does is amplitude-response modifications, though, but it's one important component, and it certainly can't eliminate earpiece reflections. So independent of the desired sonic balance, in your case I would try the mod anyway and then start from there. The more so as it has a beneficial effect on the relation between mid-treble and upper treble. To my ears the original mid-treble emphasis has indeed somewhat outshone the upper treble – a phenomenon that's almost gone since the mod, not least because the 6.3-kHz peak now is perceivedly reduced to almost zero. Add to this the nice sparkle the Silver Dragon adds to the upper treble...
The only solution that comes to mind – if after the mod the result is still on the dark side for your ears – is active equalizing. Have you already tried different earpiece placements? I wear my HD 800 very low for optimal results.
JaZZ, thank you very much for these insights. I don't really understand "parasitic transient corruption", but I do get the point about "a beneficial effect on the relation between the mid-treble and the treble", because reflections must produce some artificial enhancements that simply don't match the natural frequency outline of a voice, for example. Driven by the Meridian (which I feel REAL lucky to have - I got it quite serendipitously at a deep discount), the LA7000s on the RP010 sound like a live mike feed to my ears on certain discs, but the timbral inaccuracy of the HD800s is so contrary to its delicious imaging and amazing clarity, that I just don't get it. I'm about ready to order another pair just to see if I get a "bright" one! And yes, I have tried different positions on my head/ears, which I agree can make interesting differences.
But I will indeed try your suggestion first. What do you think -- melamine, Pima cotton or black velvet in my case?