Well, after a couple of weeks, I am enamored of these "new" "normal bias" 507's.
I think I am going to leave them the way they are until they pop, I can't think of any reason to change them unless they fail again.
They are giving me the best headphone experience I have had so far. Hugely expansive, utterly smooth, absurdly lucid, deep and subtle bass when used with my dynaco upgraded srd 7 transformer box, 300b amp, and normal bias.
The high frequencies are airy and spacious, I haven't heard anything like it. The "new" 507's make the traditional normal bias Lambda and 5n sound a bit closed in and compressed by comparison, and less detailed.
I was not happy when the original 507's failed, but now I am almost happy they did, because I never would have tried this otherwise.
I heard some of the other Stax at the headphone show. HE60 with prototype Liquid Lightening, 007 with Woo 5/srd7. Those two sounded good, but a bit inefficient and thick.
Orpheus HE90 was sweet and seductive tonally, and pillowy with its own amp, but not a detail champ.
009 with Blue Hawaii variant was extremely detailed, but a bit edgy and sharp, which is a risk of an OTL tube amp with solid state power supply.
These were limited listening with show conditions, so I am sure I would not have exactly the same opinion on extended listening, but I wasn't nearly as bowled over by them a I thought. I am sure other listeners would have entirely different tastes and impressions.
The "new" 507 seem to have a great balance of characteristics so far, I find them preferable to the original 507's when they were working.
I am shocked because I was just hoping to get them working decently again at a standard Lambda type performance and instead got a new headphone setup that I really like.
Of course, they could explode any day, but so far they are holding up. I actually would not mind re-doing the mylar every couple of months or so if it means maintaining them at this performance level.
I guess I will report back if I try to change the membranes out or they fail but so far so good on both the transformer upgrade and the mylar experiment.
More than half the time my DIY stuff is a total wipeout, so this was a nice surprise.