If passives were replaced, it's not necessarily a bad thing. The new passives more often than not perform better than the originals given the age of the amp.
I have sour experience Once, But good that i learned from it.
1.Just Remove the top cover, Dont hesitate to allow your nose to have a investigation to find any Burnt smell or some fresh plastic Smell.
2.Just have a look on the IC and the Heat sinks and check for any pieces or Lead or any color changes are there in Components.
3.Check the Knobs and button very slowly to check whether they get into rough area(When you turn then fast, you may miss the catch)
4.Just play the music and keep the speakers near to you and with very minimal volume. then follow the below testings
1.Check whether the sound is delivered with right balance
2.Turn the Balance Knob to either ends and check whether any leakage is observed to the other speakers
3.Just Mute the Sound and just drive the Volume Knob to the Max and check whether any Hiss\Buzz sound is noticed.
4.Just bring the TREBLE and BASS to the "Zero" and check whether the quality is fine.
5.Last but not least.Just have a look on the Bottom of the Amp(Foot Rest) and check for any corrosion\and damage has been identified.
I am not sure , How it would be useful, But this was usefull for me once

hyeah: