Very interesting - I've somehow left the filter tin at home today, but will try out the black backs when I get home. I've avoided them so far because I do like a bit of bass but keen to give them a try having read your take on them (may stay with black front for now though!)
On a different note, just borrowed the pair of Shozy Zeros that I'd given to a friend/colleague to compare soundstage. (Bear in mind this is red back, black front, spiral dots on the A4s)
I think the Zeros still just about have the edge on soundstage though, to be honest, I'm not 100% sure. Sounds like they're doing something funky with the positioning. But at least in terms of width, I think the difference is pretty minimal between the two.
Signature on the Zeros was pretty tough to listen to having had the A4s. A real lack of detail all round, in comparison. I think if one hasn't heard substantially better than the Zeros, they sound quite nice. But it's a little like going from a modern LED TV back to an old CRT set. The picture's all there, and it's not terrible, just a lot less defined in all aspects.
Given the difference in price, I think that's a good thing - I'd be a bit disappointed if there was little difference between the two earphones!