I use 40 foot single ended runs of Purist Dominus RLS interconnect between my preamp and mono amps. However, I do agree with the above comments about capacitance with tube gear, and with the cables needing to be the best quality for this long run. I am using a tube preamp and mono tube amps, and have not experienced any problems. Note though, that the Dominus has very low capacitance, less than 12pf per foot. This makes the 40 foot run less than 475 pf total! Not enough to cause the high frequency roll off that could occur with some cables. You can find the capacitance of your chosen cables from a specification sheet, or call the manufacturer for the numbers. The input impedance of your amps plays a role too. The higher the input impedance of your amps, the longer the run can be before it becomes a problem. Most are 47K ohms, 100K ohms is easier to drive for a long run. These numbers are not relevant for balanced, as a true balanced load should be 600 ohms. Atmasphere for instance, is a truly balanced load, and is 600 ohms on the balanced jacks, but is 100K ohms on the single ended RCA inputs. An excellent example of design to avoid this type of problem. (PS, I do not use Atmasphere, just a comment.)
