Long interconnect cables

XLR for long runs over RCA

I do not know the length threshold however

maybe someone else does

Is there a certain distance above which it may affect the sound?

A generally applicable threshold can't be defined, because many system dependent variables are involved. IMO, though, 10 feet is unlikely to be a problem in most systems.

Relevant variables include the output impedance of the component driving the cable; the capacitance per unit length of the particular cable; whether the interface is balanced (xlr) or unbalanced (rca); the susceptibility of the particular components to ground loop issues (particularly for unbalanced interconnections); the resistance of the shield or other return conductor in the cable (particularly for unbalanced interconnections); the rfi/emi environment; and the synergy or lack thereof between the sonic effects of the cable and the overall sonic character of the system.

Regards,
-- Al

I run 25 foot balanced to my front amps, not problem.

The are a variety of factors involved in your question. The single most important question is related to the output impedence of your preamp. In short, the lower the better. Long cables need current from source to drive the signal. The input impedence of your amps are also a factor, and there is a ratio between output Q of preamp to input Q of amp that is ideal. The damping factor of amp can also be a factor. Today, generally, 50 ohms is considered a low output Q. (Personally, I like lower still. There is no substitute for current or horsepower...) You should have no problem driving up to a 10 meter length of single ended (RCA) cable if the output of your preamp is in the 50 ohm range and the input of your amps are in 50 - 125K range. As a final thought, you will also want to keep the capacitance of the cable relatively low.
In this context, I'm driving a 7.5 M S/E cable effortlessly.
Hope this helps.

I run 35 feet of balance XLR's from preamp to amps, no problem.