It is most likely the dimple version are 5998 since up-specking was common practice , that is where all the parameters exceed the requirement for a given tube spec a substitute is made Very common with rectifiers

Don't overlook the GE 6as7GA it has been the bass king in all the compatible amps I have owned

I think single sided is just a weight reduction thing , maybe cheaper too

It would struggle to provide sufficient current at high demand frequencies Surface-mount op amps with minimum capacitor back up are not ideal for low efficiency

It would have been good to expand the already absurd flexibility of Ultimate Eric builds an excellent parallel 6as7 amp but it must be reserved for headphones with sensitivity 90dB or lower to avoid noise

"Balanced is not better per se unless you are running studio length cables." This is confusing balanced transmission with balanced drive, two different things