How long should speaker cables be?

Naim recommends 3.5 meters minimum for proper inductance and capacitance.

 

Naim is an unusual situation. They traditionally designed their amplifiers with a certain amount of impedance expected in the cables. Their older amplifiers could become unstable without it. Amps built by everyone else and pretty sure the newer Naim amps do not need this to be stable so this recommendation does not apply.

In regards to shorter is better, I once had a set of monoblocks and took it to the extreme by configuring it all so the speaker terminals were very close to the amp binding posts and used very short jumpers, a few inches. It sounded horrible. Shrill and basically unlistenable. Longer cables made it sound great. I repeated it several times and there was no doubt a huge difference.

I only bring it up to reinforce what many others have said.... it depends. The only way to know is to try it.... I know that isn’t much help but it is the reality

negative output impedance

please explain, I've never heard that term

Yes, as many have stated, short runs of loudspeaker cable are often best.  To be practical I would say 3 meters or shorter if possible.  But the real world often dictates otherwise.

I would like to offer that keeping single ended interconnects (SEI) to 1 meter or so is not a hard or fast rule.  This is dependent on several factors.  My preamplifier has an output impedance of a rather low 200 ohms and is capable of driving long SEI if required.  In my initial room setup, I had interconnects to my mono blocks about 2 meters long.  After changing my room, I now have a 25-foot pair of interconnects to my mono blocks with absolutely no issue(s). All my other interconnects are 1-meter pairs and a 2.5-meter pair.

Some people who have gone all digital without a stand-alone preamplifier may use balanced XLR interconnects.  

Just making this point for whatever it may be worth.