@Bojcha: I'm finally starting to make some progress on the declipper. The whole week I've been trying to use image processing algorithms on a display of the waveform to detect certain features....

But in the end it doesn't seem to help much.
Now instead I just figured out a really simple filtering method to detect clipping. And this one often detects far LESS samples than the method I'm using so far, among others in DBN - Jack is Back, but not in the Metallica track
So I'm now planning to use both methods and only consider samples as clipped if BOTH methods say that they are.
This won't fix everything, but at least for Jack is Back the result is much closer to the original. (Actually I'm now thinking that it might even be possible to combine the two methods in a smarter way, to improve things even further, but I'm not going to do that for version 6.20 anymore unless there are still tracks with issues after I've made this fix.)
@Gtxx: The GUI thread is in a separate thread. And for FM output, the FM Overdrive part is too.
@camclone: Thanks for the offer

But I don't think I can...