Very simple, when pushing one inferior variable to the best possible, another one will take the place of being the disturbing one... the one that spoils all the fun and excitement.
Well, this time, it was the stereo that was driving the fun, as I thought.
This "feature" was audible only with my PVC Voigt pipe-type speakers. What do we learn from this?!
- When using a cheap stereo, use the speakers supplied, so you don't hear the deficiencies!
- When hooking up a cheap stereo to some decent sound converters, you may be able to hear the defects the respective sound converter has.
- Dimensions given by your sound-Guru are relative... does a 1.5m Voigt pipe sound better than a 2+m Voigt pipe?! Probably not... shorter pipe, less bass..
Remedy for the hizzzzzz problem: build a dedicated AMP (opamps/LM368) in a classical way...