RCA to RCA versus RCA to XLR

Without seeing the preamp schematic I can't say for sure but it is likely you'd get an improvement in ground noise rejection as long as ground is connected to only the cold wire (XLR pin 3) at the RCA end. This may come at the cost of increased thermal noise, again depending on the input architecture of the amp. If you have no issues with ground noise then you may be best sticking with unbalanced.

I try to explain thing simply.

Thanks for the simple explanation, I'll try to explain as simply as I can why you are not correct. If you connect ground to pin 3 then it will be inverted at the input and there will be some benefit of the balanced common mode rejection (summing of inverted interference with signal interference). In addition to this the ground signal will see the input impedance which will vastly reduce the amount of current flowing through it... current flowing through ground is the definition of a ground loop.

@oldhvymec

 is there a bit of "different" thinking going on.

Yep, there's always more than one way to skin a cat... I was just addressing the comments saying there there was no benefit going RCA to XLR. If you have no issues with RF, ground noise etc. then RCA in most cases is the way to go. With ground loops there's a specific problem where current flows between grounds on different circuits, it just so happens that RCA to XLR stops this... it wasn't even the primary reason for balanced interconnects as far as I know.
For all I know a ground loop eliminator might work great, there are always options out there.

I think shielding like most things is down to circumstances, I've got wifi with around 30 devices attached (god knows what they all are). So I can't do without it, the only issue I've had is TT interconnects as the capacitance adds to the cart loading so low capacitance interconnects where possible. If you're in a low noise environment then you're lucky and can get away with trying some other options.