Is there an affordable long run XLR preamp to amp cable out there?

I agree with the Canare microphone cable recommendation above. I had HAVE, Inc. assemble a set balanced IC's (~15' lengths) with premium grade Nuetrik XLR connectors at a decent price.

I’m very happy with 25-foot cables made with Mogami W2549 and Neutrik XLRs, put together to order by Pro Audio LA. (I think the pair ran $105, including freight.) I recently auditioned some "audiophile" $2200 cables and they were not nearly as good sounding as the Mogami cable.

These run between preamp and amp in my system.

Tip: Most pro audio shops (including the one listed) can put red rings onto the XLRs of one cable for only a couple of bucks more.

Whatever Ralph and Victor recommend should work. They make differential balanced audio gear in the form of Atmasphere and BAT products.

Guitar Center stocks the Mogami Gold balanced cables (with Neutrik XLR jacks) in a lot of different lengths, 3ft. to 50ft ($39.95 to $109.95). A 25 footer is $69.95. That microphone cable is the standard in recording studios.

Van den Hull. That's what Pink Floyd used when they finally figured out that cables mattered. Don't know what model.
Atma-Sphere and BAT opinions are of little value unless you use their equipment.
But if you decide to try a pro cable also give DiMarzio a try. Yngwie Malmsteen uses it with his guitars. I tried RCAs only not XLR - OK but nothing special, ridiculously inadequate compared to some Purist and Tchernov that I have.