At what distance are balanced XLR cables needed?

Eldartford - you can spend arm and a leg for a balanced IC as well. They're also made of fancy materials - both metals and insulation. There is no reason why it should be different IMHO. Purity of metal still applies since impurity reside between crystals while insulation material directly affects capacitance and dielectric absorption.

Al - I'm not sure if metal purity or dielectric absorption or capacitance play role in any particular setup either, but if they do - I can't see any reason why it would be different for balanced vs unbalanced cable.

Balanced versions are always slightly more expensive. If noise is the only concern balanced cable can be more effective for less money. Shield on single ended cable protects from capacitive coupling and high frequency EMI, but might be less then perfect in lower range of the radio frequencies where it works still as an antenna (poor one) while skin effect (that causes induced EMI to run on the outside - shield)is not strong yet. Balanced configuration suppresses common mode signals in this range of frequencies, while twisted conductors provide further noise rejection (cancellation thru even exposure).

Kal, Absolutely. Why not to use balanced if environment is known to be noisy. My Rowland 102, a class D amp, has only balanced input connector - a mature decision on a side of Jeff Rowland. Balanced has also two practical advantages - it won't disconnect (latch) and prevents touching input (exposed side always female with recessed sockets). The only disadvantage I know of is a little higher price.

Atmasphere, unfortunately I don't like gain at 60ohm position. Not many solid state amps have 60ohm output impedance while the most of them cannot drive 600ohm. The reason for that are op-amps used. Only few like LM4562 or OPA1611/12 are designed specifically for that. In addition most of preamps have output capacitor not designed for 600ohm load. I used to solder resistors left and right when I was young. Now I know better and do always whatever is "standard".

"cost of the cable will have no bearing on the sound if"

Can you explain why purity of metal becomes unimportant in balanced configuration?

I would suspect that bigger current would cause bigger voltage drops on said junctions (or impurities) everything else being equal.

600ohm must be not very common since I've never seen anything that can drive 600ohm without distortion. For now I will stay with my Acoustic Zen Absolute XLR (7N silver, 0.02uH/ft, 6pF/ft). Acoustic Zen as well as Audioquest and many others must believe that metal makes a difference since they use 99.99999% pure copper and silver.

Eldartford, 600ohm is output impedance and 600ohm is input impedance of power amp. Because of 2 to 1 division of the voltage output voltage of the driver (driving 1.2k total) has to be 2x higher - being equivalent to nominal signal driving 600ohms. Even if gain increase is on the power amp side driving 1.2k is not easy. Many preamps have hard time with 10x that. For instance Rowland in my 102 power amp uses additional amp just to increase input impedance from 10k to 40k.

"It makes a difference in what they can charge for the product, that much appears to be true."

Drubin, Absolutely - but on the other hand nothing suppose to make audible difference, according to calculations, but this cable is way better than very decent AQ King Cobra I had before (and it is only 0.5m XLR). What make clarity so much better? Dielectric? Metal? Who knows. I think Acoustic Zen knows (and they charge an arm and a leg for that).

"The 99.99999% figure quoted for purity is outright preposterous"

Nordost uses 99.999999% pure copper.

"BTW this is why you don't see copper wire that is Teflon insulated."

I just look at my Satori speaker cables that are 6N copper in Teflon. You can get many Nordost cables that are 8N copper (silver plated) in Teflon insulation as well as Neotech hookup wire that are 7N continuous cast copper in teflon (no plating). Many Audioquest cables (like Colorado or Gibraltar) are 6N copper in Teflon.

I know about microphone standard but am unfamiliar with preamps designed to have 600ohm output impedance to drive 600ohm load.

"Actually if you work the math the opposite is true- with no current at all then the minor resistances, odd diode effects and the like will become more prominent."

I don't see why - where there is no current resistance is unimportant. 1ohm total (of all impurities) resistance with 40k load impedance divides voltage by 1/40001 but with 600ohm load it is 1/601 divider. If there is any diode effect then junction voltage increases with the current.

Output impedance is not that important within certain limits. My Benchmark DAC1 has output impedance values from 60 to 1600ohm dependent on output divider jumper position. Benchmark recommends for 1600ohm output maximum cable length of 26ft - assuming typical 32pF/ft and 0.1dB drop at 20kHz. With my cable (6pF/ft) I could go 139ft.