I'm very appreciative of your honest impressions.
Am I right to assume that the Yamamoto makes the lower treble even more forward? I seems that I may not need more than what the WA6SE provides. What I have been looking for was a way to tame the low treble and high mids emphasis. I have become rather sensitive to treble lately due to not listening to music as much. I fear pushing the low treble out more would just enter shouting territory for me. But you are right. After the switch to 6EW7s, my W1000 definitely haven't been very congested. The 6EW7s make it easier for me to listen to the W1000s right now and the lower treble with them is more tamed. I feel, however, that a tad bit of sparkle was lost.
I'd be inclined at the moment to say that the Yama does not bring the mids and lower treble forward. I think what it does is to add a rich smoothness to the tonality of the sound overall. In my system right now, I have not noticed an increase in that shouty quality. Its still there per se, because I think that's the part of the sound frequency brought forward by the headphone.
My limited experience with the Yama at this point, shows that it handles treble very well. What kind of sparkle are you referring to, any examples? For instance take the Song "I'm an Old Cowhand, from Sonny Rollins' Way Out West. In the background there is a steady rhythm made by light taps on a cymbal that runs through the whole song. The W1000 brings that cymbal forward, and it's sound is clear, refined, and just a bit lively. But of course, that Rollin's CD is of a small ensemble recorded fairly intimately.
BTW, the W1000 does bring that cymbal forward a little more that does the W5000, and likewise with the W5000 to the HD600.