At what distance are balanced XLR cables needed?

Kijanki.. And you can pay an arm and a leg for little stones.
Atmasphere, whose opinion carries a lot of weight, has suggested that fancy wire is of little value in balanced interconnects.

Jafox... I also ran long single ended interconnects for many years without problems. And they weren't any exotic wire. I changed over to balanced interconnects simply because I bought some new equipment having this interface.

I trust Atmasphere on this subject not only because he is usually well informed but also because he once provided a good writeup on the subject of balanced interconnects. Maybe he will come by and do it again.

If the HT Processor output impedance is about 50 ohms or less (typical for solid state) 20 feet should work fine.

If Zone 2 is really 51 ohms you could use that output. 300 ohms is a bit high for solid state but it will probably be OK.

Bear in mind that you can spend an arm and a leg for unbalanced interconnects made with exotic wire. Balanced interconnects don't need exotic wire, and might cost you less.

Kijanki... 600 ohms is the output impedance of the preamp. You don't have to drive 600 ohms.

200K is the input impedance of Atmasphere power amps.

Kijanki... I have a nice bridge in Brooklyn. Would you be interested?

Atmasphere... Question... 600 ohms is a practical low output impedance for tube circuits. (Some are 1000 ohms or more). But solid state circuits can be much lower eg: 50 ohms is common. Is the 600 ohm value a maximum?

Atmasphere... So the 100 ohm output, balanced, from my Behringer DEQ2496 is OK. Whew!!

Your preamps are no doubt better than the average tube gear. 600 ohms is what I always saw with the mid fi stuff I used.