60 cycle hum

I'm running a Rega P5 with outboard PSU into a Yamaha receiver with an internal phono stage. I hear no extraneous noise with the needle off the record 'til the volume control hits 12:00 - it then increases as the volume increases.

Playing records, the Exact cartridge has such a high output that I routinely listen between 9:00 and 10:00 and get plenty of volume...

Is the hum a problem or am I looking for an excuse to buy an add on phono stage?

BTW I seem to recall the same "problem" with an old HK receiver and a Denon TT with a Grado cart........but I thought it was Grado hum and when I swapped the cartridge for an Ortofon it was still there, but only at very high, not listening, volumes.

any comments would be appreciated.
Cheap aint nutin to do wit notin where hum is concerned. Anyway Yamaha aint cheap, its mid-fi. And I never heard them do that. But 60 cycle is a cooky animus. You could try breaking it all down and re-connecting. I do not know Rega by experience, only reputation, which is relatively very good; does it have a ground wire? Try a cheater plug to break a ground loop (if there is one). First the receiver to the wall, then the table.
I had to get 8 foot grounding rods from Radio Shack to fix mine. And if it does not rain in two months, even that does not work.
Failing that, borrow somebody's receiver. If that works it is probably too expensive to repair your Yamaha. Get a Rotel if you need the tuner, else a Creek, or even an Onkyo.
Aceto has good idea and make sure that cables areb b not too close to each other or power supplies.Think PS Audioo make a device called humbuster but not sure how it would work with yout dry up.
High output cartrdiges like the Exact are going to have some "noise" associated with them when you crank the volume on your amp up.

I'm a bit perplexed by the responses from Aceto and Readster. Your analog input is never going to be as quiet as, say, your CD input. It's not a problem though unless it's above the noise floor of your cartridge when playing.
Additional information: hum present in both channels (speakers). Becomes louder as tonearm approaches the spindle. Hum disappears when phono leads disconnected from receiver. Doesn't seem to vary when leads moved away from/toward power cords behind receiver.
Are you using a VPI Power Line Conditioner to fine tune your speed? The old PLC would cause hum if placed too close to circuitry. Moving it away a few feet would solve this.