Hi eric65, I'm not disagreeing with most of your post, but the above tube advice is plain misleading IMO.
It gives the impression that the stock tubes have the correct balance and that the only reason to go for alternatives is to introduce euphoric imperfections to counteract the particular flaws of certain headphones. This is very much not the case. Tubes like the PHs sound fundamentally better than stock. They are more perfect, rather like inserting a better source that will improve things like image solidity and focus. There may also be tonal characteristics as well, but with the models I've tried, these tonal characteristics are by-products of the fundamental improvements. I'm sure all the tubes will have a ruler flat frequency response compared to headhones. The tube differences go well beyond a simple tone control.
With the proviso that some people don't seem to hear much difference in the first place, when you try it yourself, you'll see what I mean. The alternative tubes sound better because they are better, not just because of some tonal imperfection has been added. The tonal by-products of these fundamental improvements may well synergise well with certain headphones, but by and large the improvements will benefit any headphone.