Upon inspection of the circuit board, I was surprised to find what may have been the culprit. In the LPF section, a JRC4558DD was used, yes it has a double D (the inferior version of the JRC4558D, duh!), and a ROHM BA15218 in the output buffer.
Datasheet shows that the BA15218 is an excellent opamp during it's time but the JRC4558DD is just so full of %$#@. I try not to think about it but this player wasn't cheap when I bought it.
So with a warmed-up soldering iron and de-soldering tool, off they went! As always, psu bypassing took most of the tweak time.
I encircled the lpf opamp in red and the buffer in yellow, I tried several opamps from Motorola, On Semi, Analog Devices (AD), Burr-Brown, etc.
I was itrigued with the AD opamps since they were renowned to be tube sounding. However, when I used AD chips on both lpf and buffer stages, the sound lost it's air, the highs were so rolled-off! When I tried Burr-Brown, the highs and mids were excellent but kinda "manipis", so I ended up mixing brands until I got what I want.
Can't believe it, mataba na may air at walang sibilance ang result.
It wasn't a waste of time after all.