Counterpoint SA-5000 - phono stage question

I've had the Counterpoint SA-5000 for many years. Just recently got back into vinyl and while the overall sound is fantastic, there is a hiss that begins just past 12 O'Clock on the volume dial. It's not audible at my normal listening levels, but I want to get to the bottom of it.

Seems to me that it is coming from the MC side of the phono stage (my turntable has an MM cartridge). It's not from the turntable itself, because I unplugged the RCAs as well as the ground and its still there when I select MM (and of course on MC). Worse, with MM selected I played around with the MC load dial and I could hear the click loudly through the speakers. In fact, which MM selected, I get the least noise when I select the least load (100) and the hiss gets progressively louder as I increase the MC load. All this while MM is selected, so the MC side of things is obviously the culprit here.

Would be grateful if anyone could shed some light on this and possible solutions. The one solution that has been suggested to me is to short circuit the MC by using shorting RCAs - is this safe?

When was the last time you replaced the tubes? Since you haven't used the phono side of the preamp for a while, it might need new tubes. The tubes are still powered up and have current running through them even though you aren't using that input. I'd start there.
What Mofimadness said.
Get new phono tubes in there. They have to be selected for 'low noise' so be prepared to shop around and pay more than the usual.
Thanks - I changed the tubes just recently and the hiss did go down quite a bit, but not gone altogether.

I want to try and short the MC to see if it goes dead quite, but would love to hear if its safe to do so.

(from what I can gather, the MC stage on this Counterpoint, unmodified. is not the finest and I don't plan to use it in the foreseeable future anyway).
By the way - the tubes I got were Refleckor 6H23N-EB/6922

Replaced all four (phono and line - the ones in phono stage were original stock, had "Counterpoint written on them!) and also changed power tubes (Electro Harmonix). The tubes made a huge difference overall - but that hiss from MC (less, but still there), persists.
Got the shorting RCA plugs I had ordered - hiss is almost completely gone. I can hear it still with the volume dial 3/4ths of the way, but that is way beyond any listening level I'd be driving it to. The hiss is now gone when turning the gain dial too.

The other thing that made big difference was leaving the tubes on 24/7. Might be partly due to some breaking in, but the preamp has now been on for the past week (standby mode when not listening) and that has made a significant difference in overall sound as well.
I had an SA 3000, it sounded great but tended to be a bit noisy...micro phonic tubes and a bit of hiss no matter what I did. I finally got used to the slight noise and just enjoyed the music.
Hello. As owner manual says, you must plug the shorting RCAs on the unused phono inputs. I also have the same problem with my SA-5000 so I'm thinking to change all the tubes, even the rectifier in the power supply chassis. I'd like to upgrade some other components as capacitors, for example. Do someone knows if it is possible to have the schematics of Mr. Elliot upgrade? Thanks, Luca