I suspect the problem is your extremely unusual choice of amp and speakers.
First Watt F1 is a current power amplifier...it will attempt to drive your speakers with a current that is independent of impedance load.
Your speakers only have one driver...it is unlikely that the impedance curve is anywhere near flat ....even it might be an easy or efficient load to drive.
I don't know the impedance characteristics of your speakers but most speakers are such that they tend to be of higher impedance at higher frequencies and usually high around the resonance point of the driver (which for 4 inch is probably some where in the lower mid range or bass).
The end result of this combination is that you will accentuate the speaker response according to the impedance curve...i.e. emphasize the resonance frequency and accentuate the treble (if your speaker behaves as most speakers do) => you will get significant bass and treble response boost and this is why you hear so much sibilance
Current amplifiers are rarely used in audio applications for this very reason....they need to be carefully matched to a speaker with a flat impedance curve.
First Watt F1 is a current power amplifier...it will attempt to drive your speakers with a current that is independent of impedance load.
Your speakers only have one driver...it is unlikely that the impedance curve is anywhere near flat ....even it might be an easy or efficient load to drive.
I don't know the impedance characteristics of your speakers but most speakers are such that they tend to be of higher impedance at higher frequencies and usually high around the resonance point of the driver (which for 4 inch is probably some where in the lower mid range or bass).
The end result of this combination is that you will accentuate the speaker response according to the impedance curve...i.e. emphasize the resonance frequency and accentuate the treble (if your speaker behaves as most speakers do) => you will get significant bass and treble response boost and this is why you hear so much sibilance
Current amplifiers are rarely used in audio applications for this very reason....they need to be carefully matched to a speaker with a flat impedance curve.