Longest Advisable Speaker Cable Runs?

Look into Supra and OCOS cables. I have used both for runs of 18 ft. with no issues. The Supra can be purchased through
Sjofn-HiFi (get the shielded set). The cost and return policy make trying these very low risk. Not sure who is selling OCOS these days but they are considerably more expensive, unless you can track down a used set, and I don't know if any OSOS dealers offer a return policy.

I think Al has summed it up best in his last post. Regarding the Supra cables I believe the ones I used were the Classic series, although I don't recall exactly. Yes the shielding helps but the other parameters Al mentioned are more important. As for OCOS, they market themselves as the cable to use for long runs. I don't exactly get why from technical perspective, but I know it has something to do with their zobel networks that are available for use with the cables. There is a review on this in 6moons if you want to read up on it. I do not use the zobels with mine, but they may be beneficial with solid state gear.

For Supra cables if you are in the US give these guys a call:

Sjofn HiFi

If you click the link you will see that these cables are not that expensive. The set I bought was $26/meter per pair (S/Ply 3.4) but for $12/meter per pair you can get the Classic 2.5 that are also very good. You need about 9m so if you go with the Ply 3.4 it would be about $250/pair and if you go with the Classic 2.5 just over $100.

As for OCOS, Dynaudio now represents them and you can call the directly for pricing. Yes they will be pricier, but again you can look for them used.

I guess another low cost alternative could be Blue Jeans cable. Many here on the forum seem to like that company.

Thanks for the correction Al. Need to use the calculator next time. Still quite reasonable for such a long run.