40ft: speaker cable or interconnect?

I recommend balanced interconnects when using long runs. Single ended ones suffer from high frequency roll-off and also can be much more susceptible to noise, which are significant reasons why balanced connections used exclusively in professional applications.

John, long single ended *is* possible. I solved the problem I was having (described above) by moving to a new amp (which I was coincidentally looking to do anyway). My new amp is an AVA FetValve, which has a very high (and stable) input imdedance and makes for an easy load to drive with long ICs. The AVA only has RCA in, so I tried a few things and went with well shielded RCA ICs (Purist Venustas) that are 30 ft long. I compared them to what I was using in my old arrangement (Kimber KCAG at 1.5m) and discovered no discernable rolloff or ill effects in using the Purist cables. The resulting combination works very well for me. YMMV, and I would suggest being very careful to assess all parts of the system in trying to find out if this might work with your equipment - even to where the cables will be routed relative to other sources of potential interference. But it can work.

My results are identical to Tonyptony. I have nearly 40 foot runs between my tube preamp and amps. The amps are VTL 750 (single ended design) so I use RCA connectors at both ends.

Just because an amp has XLR connectors does NOT mean it's balanced.

Many amps have both types of input jack but pulling the cover you discover there are jumpers running from the XLR over to the RCA.

At that point it's merely a convenience jack, providing a place for the owner to use an existing cable. There is no gain to be had wiring the circuit this way, unless you just happen to prefer the XLR connector over the RCA design.

The trick to maintaining performance is choosing LOW IMPEDANCE, low loss interconnect cables. The reason Tony had positive results, the Venustas he choose are approximately 17 pf per meter, meaning a 50 foot single ended run represents a capacitive load of only 250 pf.

I don't have the charts or specs for the input impedance of your amp/preamp, but I'll bet roll off would not occur until about 300,000 kHz. That’s far beyond the range of doing harm to music bandwidth and should provide you with the lesser of the two evils of running signal over a long path.

Whatever brand of cable you choose, be sure the impedance of your amp/preamp interface COMBINED with the total pf capacitance of the cable run do not represent a bandwidth problem. Most engineers understand this, you might call an impartial party such as the maker of your amps for specs and advice for maximum load that can be placed on your equipment.

On doing some more thinking, I've concluded that I really do not want to locate my amp/amps up near the speakers. If this were a 2-channel listening-only system, I wouldn't have a problem. But since it is my combined 2-channel, HT rig in a dedicated theater room, I really want to keep things as clean and open in front as possible.

I did some research and found Roger Russel's page on cable myths. I won't get into the high-end v. lampcord debate, but what I did find useful was his chart showing cable guage/speaker impedance/length interaction. It tells me that if I stick with a good 12 or 14 guage cable, I really should have no problems and not notice any rolloff. If I go to a 10 guage, I gain even more 'breathing room' in the length.

Unless you guys think I'm making a big mistake, I'm probably going to get some of Frank's Ultra Run which are 10 guage or the Audioquest Bedrock which are 12 guage. Frank's cables are always a good deal and the AQ is on special right now at AA.

You could try Sumiko's OCOS cable, which is constant impedence no matter how long the run. It was at one time a Stereophile "Recommended Component". I'm using it as biwired runs of about 40ft to Vandersteen 5's from a SMc modified DNA225.