Hum with Cary PH-302 phono amp

Hi All:
I know there have been a lot of threads relating to hum here on Audiogon, but I haven't seen one like this. I wonder if someone could help.

For several years I have been using a phono set up with a Zyx cartridge (.24 mv), Zyx pre-preamp (26 db gain) into an Audio Experience tube phono amp. Tonearm is grounded to the Zyx pre-preamp, then interconnects to the phono pre. No problems.

I recently upgraded the phono preamp to a Cary Audio PH-302, purchased here on Audiogon. I initially ran the Zyx cart through the mc phono stage of the Cary. All worked fine. I also tried a high output Benz through the mm stage and all worked fine.

Here is the problem: I want to compare the Cary mc stage with the ZYX pre-preamp through the mm stage, but when I connected the tonearm and ground wire to the pre-preamp and then connected to the Cary (mm stage, of course)I got a very loud hum. I ran some wire from the grounding screw of the pre-preamp to the grounding screw of the Cary and this lowered the hum considerably. Still, the noise is present and loud enough to intrude on soft passages of classical music.

I've tried everything I could think of:
1. I tried the ground float switch on the Cary both ways. Even tried cheater plugs.
2. I placed shorting plugs on the unused phono inputs.
3. I tried every grounding combination possible (tonearm directly to Cary, then on to Zyx, etc)
4. I tried reversing the interconnects, even tried a different brand.
5. I reconnected the ZYX pre-preamp to the Audio Experience to make sure the unit had not developed a hum and it worked fine.

I'm out of ideas. Both the Audio Experience and the Cary mm section are 47 Kohm input and the Zyx pre-preamp is designed for 47 Kohm. I can't see any reason it shouldn't work. I spoke to tech support at Cary but they had no answers. Their solution was the obvious one: just use the mc section of the Cary. Well, I might end up doing that, but the ZYX is a nice piece of equipment and I would very much like to be able to compare the options before making a decision. I expect many of you would feel the same.

So if you have any ideas, or if you have had a similar experience with a pre-preamp/phono amp combination I would love to hear from you.

Thanks in advance,

What is the gain of the Audio Experience versus the Cary MM? You could be just overloading the Cary and it's being induced as hum.
Gain on Cary is actually lower. Stated at 38db. 26db on the ZYX + 38db on the Cary. Should be almost perfect for .24mv output, right? I believe Audio Experience is 54 db. I really had too much gain.
Yea, that goes backwards to my thinking...which really isn't new :-) It's a mystery.