I have had a testsession on some Stax and Audeze gear last week at a Hifi Store in Germany.
I own the 303 classic headphones with the SRM-323II. I have had a few hours with the 507,
007 and 009. Here's what I think:
009: What can I say. I was disappointed with the sound signature. Extremely foreward and very agressive.
Perhaps good for Classical music. It has a fanstastic heavy and dry bass but the mids and highs were too
agressive for me. Especially with louder volumes and modern music (Hiphop, Electro, etc.). And I was missing the "magic".
007: More fascinating in my opinion. A bit rolled off in the highs it's more dark and veild but never disturbing in the highs and mids.
I will have to take a closer look at them and test them for a longer period.
All impressions of the headphones where made with the new SRM-323S (apart from my own set).
That's exactly the biggest reason you were hearing what you heard. I very much dislike that amp and anyone saying it sounds like any other ToTL Stax amp hasn't heard them side by side.