AR SP-11 RCA outputs to ba lanced inputs.How?

There are few different ways:
1. Cardas RCA-XLR adapters;
2. Professional grade interconnects(Canare) that are priced cheap and perform superb for the price offered;
3. Order an audiophile grade interconnects from AuidioQuest or Cardas which will cost much more than pro-grade.

In all of the above cases the minimal loss in performance is when no adapters are being used, but if you consider buying a high quality interconnects that are terminated different you might have a hard time when you want to sell them.

There is another way...just modify the SP-11 to have XLR connectors on the output (in addition to the RCA connections)..there is room in the rear left corner. Then just use balanced cables...no degredation to this method and maximum flexibility.

the ideal case in reality when the whole system is either balanced or unbalanced to take a real advantage of canceling the noise.

for a true-balanced preamp modification you will definitely need something more than directly soldering XLR receptacles and mounting them onto the chasis. you will have to know how to separate the positive signal wire from negative signal wire and the ground to take the full advantage. simply coupling negative signal wire with the ground will bring the same result as unbalanced RCA and simply adding a parasite unused outputs.

on the other side of that issue you might never know unless you open the amp and see how XLR plug is connected there before you make a conclusion that it is true-balanced amp or any component. in this case there is no point to modify and you'd be better of with differently terminated interconnects.