The volume pot in that Melos SHA may be special ... it is light coupled or a LDR ... notice the "phot" in Photeniometer meaning light
Some feel that these light coupled volume controls are better sounding than resister based controls and most likely why Melos chose to use them
You may want to do some homework about the Melos SHA before swapping for a basic Volume control
If you are trying to upgrade with ....
.."right one of these"..
... you may end up taking a step backwards
If you are having a problem with the volume control ... you may want to Google for help