What Interconnect for Mcintosh mr-78 tuner

Try 2. Hand Audioquest Cheetah. Excellent cable.

fwiw I thought AU 24 sounded thin on my system...

I don't have your tuner but for years I have found Cardas Golden Refernce to work well with my McIntosh gear. You can buy used pairs here for 500 give or take. If you don't like them you can get your investment back. Or as another option check with usedcable.com and their lending library.

I just bought the Audioquest Diamondback for my MC2100 and they work awesome as in very clean sound NO hums. Highly recommended.

I have MR-74 instead of MR-78 but both should have similar characteristics. McIntosh tuners have very high output voltage, 2.5V vs 1.0V on most other tuners. The high output voltage makes the MR-74 sounded rather forward, a sonic character that one does not normally associate with McIntosh equipments.

Instead of using fixed outputs, I suggest you try the variable outputs. You will not loose resolution using the VAR outputs but its sound will change depends on the volume setting. I think you will find a setting that fit your system well.