There are two of those resistors in the amp. R44 and R45. They are 0.22 ohms +/- 5% tolerance, 10 watts. They are connected in parallel, one end to ground, the other to the speaker output jack sleeves and R42.
The resistors are in the power amp, not the preamp as you suspect. Look for a second resistor just like the one that fell out. Then again, if one came out perhaps both did. You may have to trace the circuit using the schematic to find them. Probably on the board that we are seeing the back of in your images
Download the schematic here:
Link Removed
This finally solved it for me. These resistors are obstructed by a support bar and I didn't see the holes on the pcb under it where this fell off. Thank you kindly!
Any idea what this white grease is? I want to wipe it off but not if it's important





