Help with Hum issues.

This is all becoming too confusing and hard to follow.

Let me state (To simplify this hopefully) how this all started and where I am now:

I started out connecting an Onkyo large Amp and matching pre-amp.  Had a hum in the R side.  Grounded it every which way, sometimes it is better than others but the hum is never gone.

I noticed the Phono stage on the Pre-amp is often faint or fades away.

So I replaced the Onkyo pre with a Phase linear pre.  I still have the hum.

So I removed the Onkyo Amp and replaced it with a Mitsubishi Amp.  The hum is still there (NO matter what source or NO source).

This is also true with the Original set up (ALL Onkyo)

So just now, I did the following:

To complicate things further or maybe not:

I removed the Phase linear pre amp and connected the Onkyo pre amp instead.  Connected to the Mitsubishi amp.   I also connected a Tuner as a source.

The hum is gone but the Onkyo pre amp has issues with the phono stage and the L, R, Mono switch....as I mentioned above.  I have cleaned the switches with the spray but the issue is not resolved.

And when I had the Onkyo amp connected to it I had the same hum issue.

So what does this mean ??

I guess the Phase Linear pre amp is causing a Hum ?

And the Onkyo Amplifier is also causing a hum ?

And I need to fix the Onkyo Pre-amp as a Minimum and maybe the Onkyo Amp if the hum doesn't go away.

Ok, I hope things are clearer now.

So I tried these different setups:

1- All Onkyo (Amp, pre-amp).  Hum in R. side.

2- Onkyo Amp, Phase Linear pre-amp.  Hum in R. side.

3- Mitsubishi Amp.  Phase linear pre-amp.  Hum in R. side.

4- Mitsubishi Amp.  Onkyo pre-amp.  Hum is gone.  But there are the issues with the pre-amp that I stated. 

I eliminated the signal source (Tuner) since it is not the issue here.

All cables and speakers were changed so those are not the issue either.

What are the odds that the Onkyo amp and P. Linear pre-amp needs repair in addition to the repairs needed on the Onkyo pre-amp ??

And would any of this cause the Hum ?   I don't understand.

Thanks Tvad 🇦🇺

Hey Customerfirst, it would seem you have tested all the configurations. Your conclusion would seem unlikely, but possible. BOTH preamps with problems?

What models are the amp/preamps? How old are they?

I would also agree with rixthetrick that there may be a dry joint or component failing in each (you need to be unlucky).

If you were talking about some sort of room treatment of a dubious source, you might attract more notice and help.

If I can think of anything further I’ll pop my head in and see how you are
doing. Good luck from me. 🇦🇺

Do you have a competent electronics repair service in your area, especially a service agent for reputable hifi brands?
Perhaps you could get on Google and look for one, especially if you're in a highly populated area.

A good service agent should be able to with the correct tools identify the problem and give you ideas on costing to repair the device/s

You may have mentioned this, and I missed it, but: Any chance there could be something going on with the speaker?