There are a whole ton of recommendations online that if you just used balanced cables that none of this is a problem, but rarely do those making this recommendation make any mention of cable capacitance. Generally the argument is that balanced cables are more immune to picking up noise (true), but this notion is rarely quantified in any meaningful way. To put it another way, it's extremely rare to have noise issues from unbalanced interconnects resolved by using balanced ones in the setting of a typical home audio system.
For zero feedback SET amps with relatively high output impedances, the negative consequences of longer speaker cables are a bit diminished. Still, it's a worthwhile experiment to buy an inexpensive 25 foot and 3 foot set of interconnects from the same manufacturer, then the same for speaker cable (just cut some off a roll perhaps), then you can listen for the differences in your system. The cheap-o cable is likely to have a lot of capacitance, so it should make the HF issues much more apparent.
