any simpler audio delay method?

As I said, cheap solutions should exist - opAmps for bandpass etc. Simple! The design solution is in various Application Notes. (Ones that are so old that even when on the web as images (as pdfs, jpgs, pngs etc) they are not text searchable...)    Your response (no pun) worries me:

nazo wrote: Well, let's also not forget that this might not even help to resolve the initial problem itself....

(but it MIGHT help!) Are you simply stating the obvious, or are you saying you have NOT done or tried that? I assumed you had a good bed to start with.... "...so this is just making things generally louder.". Hmmm. I think the reason people use the "good speaker & layout" approach is that - apart from reproducing the original sound rather than post-read sound manipulation - it is CHEAPER than the phasing method (and less likely to need constant tweaking). You are merely confirming my POV. And although cheaper processing options exist, most do not seem dedicated enough to design their EQ substituting band-pass phase shifter from opAmps. (If they do, they don't produce & sell). But I'll bet they find the right setup is good enough anyhow. You start with that anyhow. Added processing is the last resort (eg - restoring a 1984 cassette to CD quality). Even better speakers (than 3" home mids) will be cheaper than the opAmp path....