The phrase "single ended" in regards to power amplifiers is in contrast to more commonly used push-pull designs. While a single output tube can be used in purist single ended designs, they are very limited in power due to inherent power capacity of a single tube operated in such a manner. So it is common to use multiple output tubes in parallel which provide higher power output while still being single ended in operation as opposed to push-pull.
That makes sense, reminds me of PSE 2A3, 45 and EL84, which use 2 output tubes per channel. Since OTL doesn't require a output transformer, it's easier to have more tubes in parallel.