Marantz PM6005 potentiometer
When in doubt, check the service manual.

Audio signal from the input RCA jacks is routed through a LC78212 analog switch IC for source selection, then directly to the volume pot, which is shown in the center of the block diagram. 

Note this pot has an extra pin for loudness control. If you replace it with a pot which does not have this extra pin, the "loudness" button will no longer work. There might be a special part number for that, so make sure you get the right one.

Changing the pot's resistance from 50k to 100k will alter the loudness filter cutoff frequency. Might be a problem if you use it a lot, in this case you can adjust resistors and caps in the loudness circuit, ie multiply resistors by 2 so you they scale with the pot going from 50k to 100k, and divide caps values by 2 to keep the filter cutoff frequency where it was.



Now check "Volume PWB" page 34 of schematic PDF from link above. The pot's wiper goes to the preamp, which is a discrete current feedback design. If you use the "source direct" mode, the output of this preamp is routed directly to the power amp. Otherwise it goes through the tone control which uses 4580 opamps.



Since the pot's wiper only feeds two transistor bases (or 45980 opamps) it only sees a very light load. If you replace it with a 100k pot, there should be no ill effects besides a slight loss of gain due to the 56k resistor to ground at the input of the preamp. This should not be a problem, as in most amps, the volume control never goes to maximum level anyway.

Note:

You can try a fix before changing your volume pot.

If the pot is only scratchy in the "usual spot" where it spends most of its life, but the track is still new at other "way too loud" positions, you can fix the problem easily by lowering the gain of the whole amp, which means you'll have to turn the pot a little bit more to the right, thus using a portion of the track that isn't damaged. I used this trick on a TEAC amp with great success. It's dumb and low-tech but... hey, it's free and it works!

You can do this by replacing the 56k resistor at the input of the preamp with a lower value. This will form a voltage divider with the pot. It'll change the log curve a bit though. I didn't find this to be a problem.

You can also insert a resistive voltage divider after the Source Direct relay to attenuate the signal by a few dB.