That guy didn't know what he was doing and he must have been getting money under the table from suppliers for using way more parts than necessary. The cathodyne does have a very low output impedance at the cathode so a very long cable can be used for the send. But the cathodyne has half the headroom as a cathode follower for about the same output impedance. His cathodyne is nowhere near center-biased so that it uses more current than necessary and the bias will wander with large signals. The amp will function properly with no device in the loop, but it won't with a line level or less device in the loop. You would have to turn the master volume way down for line or instrument level and there is no return stage to bring the signal back up to fully drive the 6L6's. To make matters worse, there are two voltage dividers after the return jack. R33, C17, and R34 do nothing for the frequency response, but they do make the first voltage divider more severe. They may be an artifact from the reverb version.