I own Sonata III loudspeakers, and I have gone through several tube and solid state amplifiers trying to get the speakers to sound balanced and not tipped up. What I have discovered is that in my system, the Sonata III require amplification that doubles output as impedance is halved. This is the only amplification that has resulted in excellent and balanced sound, which I now have.
The Creek 5250SE does not double output as impedance is halved. It produces 60 watts into 8 ohms and 75 watts into 4 ohms. This results in the tipped up sound you are hearing because the upper frequencies are being reproduced louder than the lower frequencies, where the impedance is lower. You require an amp that produces 60 wpc into 8 ohms and 120 watts into 4 ohms for the Prelude II to sound their best.
So, in my opinion, you need to change your amplification. When you do, you will very likely hear positive results.
The Creek 5250SE does not double output as impedance is halved. It produces 60 watts into 8 ohms and 75 watts into 4 ohms. This results in the tipped up sound you are hearing because the upper frequencies are being reproduced louder than the lower frequencies, where the impedance is lower. You require an amp that produces 60 wpc into 8 ohms and 120 watts into 4 ohms for the Prelude II to sound their best.
So, in my opinion, you need to change your amplification. When you do, you will very likely hear positive results.