just curious, what is R576? if it's a ceramic resistor, and it gave you a different reading than the others like it (even though you shouldn't measure resistance with power applied), then something is different here. i would measure the voltages across those resistors with it powered up. R576 is the emitter resistor for the output transistors in the Rch. this tells me that it's R576 and the output transistors, and their associated components you should be checking. one of the output transistors could be bad, or one of the 180 ohm resistors feeding the bases of the outputs. Q554 and Q555 aren't likely to cause distortion, but if there is a problem with the output devices, it's likely one or both of these transistors is bad too. with the POWER OFF. use the ohmmeter in diode check mode to test the 2 power transistors. the picture shows the pinout of both devices. on the MN2488 from B-E and B-C you should read open with the negative (black) ohmmeter lead on B, and you should read low resistance with the red lead on B. from C-E you should read open with the red lead on C, and you MIGHT read low resistance with the black lead on C. with the MP1620, you should read low resistance B-C and B-E with the black lead on B and open with the red lead on B. you should read open C-E with the black lead on C, and again you MIGHT read low resistance with the red lead on C. if all of these readings are ok, proceed to checking the resistors with the ohmmeter back in resistance mode. the ceramic resistor R576 should read less than an ohm end-to-end (meter leads have some resistance that makes reading low resistances somewhat inaccurate). R574 and R575 should read close to 180 ohms. when testing D551, D552, D553 and D554 should all read open with the red meter lead on the end with the stripe (make sure you return to diode check mode) and low resistance with the black lead on the end with the stripe. since we're looking first for the cause of distortion, don't worry about Q554 and Q555 yet.
do these checks first, since if you don't find anything wrong here, we're headed back to IC501, and static testing of IC501 isn't so simple, but we do have identical IC's near it to make comparisons with