Help On Op Amp Options

Hi,

I am trying to improve the sound quality from the Alpine TUA-T100DAB tuner as this is the only non-digital source into my H900 processor and started trying out different op amp to substitute the NJM4580s seen. With AD8599 replacing all the NJM4580s, I get good bass but sound quality became aggressive and harsh especially for the vocal ... not sure if op amp needs burn-in period but it is not oscillating as scoped.

I checked the service manual again per enclosed and note that there are LPF stage, 2 buffer stages and this ISO amp which I am still trying to find out learnt about them.

Can anyone help to advise on the following:
- why is there 2 buffer stages as I did not see 2 buffer stage in the block diagram ?
- ISO amp would not affect sound quality and hence can be left alone ?
- any recommendation for the Op Amp replaced for the LPF stage IC5, Buffer Stage IC8 and Buffer Stage IC307 (or even ISO amp IC308) ??

Please also advise what are the consideration for choosing "better" op amps for the mentioned LPF stage, 2 buffer stages and ISO amp, if at all possible.

Thanks in advance.
 
Missed the enclosed schematic and block diagram
 

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  • TUA-T100DAB_Block Diagram.jpg
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  • TUA-T100DAB_Schematic.jpg
    TUA-T100DAB_Schematic.jpg
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