I wrote about this to you already Pier Pablo, but I have to say it here as well.
Today I was finally able to take the op amps into use.
They are just INCREDIBLE!! The Sontec EQ is a great test application because I was able to compare your DOA's to a sound I am very familiar with (OPA604). There are two opamps in series (input & output) so any bad or good effect will be easily audible. OPA604 sounds clearly harsh in comparison to your gems.
I first replaced one channel with the APP2050 5.2 version and was completely blown away. My experience has been with transformer coupled tube gear and opamps only, and I had no idea a DOA can easily rival (or better!) a high class tube sound in clarity.
To be honest, I did not expect the FET version to be remarkably better, but then I replaced the other channels OPA's. and there was certainly overall more accurate sound. I prefer the FET version of these two.
I can imagine a preamp with the APP2055 (+APP2055servo) and some high end transformers would probably have no rivals.

Now I just have to decide which one to build. Or maybe an all DOA EQ.
I will definitely need more of these in the future. Lots more.