I'm failing to understand how replacing a 500k with a 500k will result in a large difference. Granted, pots have a pretty wild tolerance (iirc +/- 20% ish), but even then I wouldn't expect drastic differences. Certainly sounds, to me, more like a possibly bad pot than anything, in that case.
I could see where a larger pot of the same resistance level would give you a longer sweep through the range, so maybe a little finer control over the volume?
