19.43 tweeter diaphragm problem

Hi everyone.
I have a pair of fostex active monitors (pm 0.5). Not the best, but still ok. After a long time I decided to change the nearly knackered tweeter diaphragms.
Unfortunately fostex sells only whole tweeters for a large sum, so I decided to buy simple cheap 19.43 ones from ebay (I couldn't find better ones anywhere). I haven't got a real idea of what the proper impedance should be, but the ones I used are 4ohms. Does impedance play any role on frequency response?
The result is that I haven't got any good treble response. The speaker have become too muddy.
I have to admit that I didn't use firm super glue, for gluing them on their plastic base, and I used a black rubber glue I've used for a different pair of woofer foam. Is this ok?
The tweeters I 've used are made from a much darker silk material comparing to the thinner almost, much finer (also silk) material of the original ones. Is this a reason?
And also there is quite a gap between the tweeter domes and the diaphragms surface.
Any ideas what have I done wrong and if there is any solution or tweak - even ideas for better diaphragms if there are any?

Thanks in advance