Phono Cart Impedance Question

I have a little bit of hiss through my 6 ohm, 0.28mV cart but only at a volume pushing the upper end of my listening volume...and it's only audible before I drop the needle, and even then not from the listening position.  I realize I therefore likely have a "non problem"!  

But I still have a generic question on to the sensitivity of producing hiss as it relates to cartridge impedance alone.  If one kept all other cart specs identical (including output of course) would you expect hiss to go up or down at the same volume and system gain if I used an 8 ohm or 12 ohm cart instead?  It seems like the hiss would likely go up just like moving to a higher impedance speaker under the same gain setup but perhaps I'm looking at this incorrectly? I realize moving to a slightly output cart could very well reduce the hiss.

Thoughts?   Sorry if this is a ridiculously obvious question and answer.

You mentioned it may be a tube issue have you tried tapping the tube pins on countertop lightly or tap glass bottle with pencil eraser. I have same issue sometimes with my broken in or newly purchased NOS tubes. Some metal flakes can be on internals of tubes and can make a light hiss or even a horrible noises. I've always been amazed by just tapping a tube can make it dead quiet again.

@jmolsberg I appreciate all of your really helpful feedback/input.

Back in the early 1980's, I had a preamp with three M/C cartridge impedance settings.  My neighbor, the factory sales rep, change one resistor to give me a 10 ohm setting.  It got rid of the hiss.
dan, possibly you got that result, because a 10-ohm load resistance would result in a very severe high frequency roll off in the audible range, using almost any cartridge except maybe a very LOMC with a 2-ohm or lower internal resistance.  You threw out the baby with the bathwater, as the saying goes.  "Hiss" is probably at about 2kHz or higher frequencies, usually.