The concept is correct, but is based on non-transformer coupled Solid state amplifier.
To describe the HA-6A circuit correctly, it is balanced input, single-ended amplification and balanced output. The HA-6A is a transformer coupled amplifier. With output transformer in the final output stage, it is fairly straight forward to "generate" a negative phase in the output signal, so both single-ended and balanced output are "original" output from the transformer, we don't take the single-end output and then convert it into balanced output like other non-transformer-coupled solid state amplifier.
On the other hand, HA-6A "converts" balanced input into single-ended before feeding the signal to 12Au7.
We opt for single-ended amplification because that's the best sounding design for tube based amplification in our opinion. For fully balanced tube amplification with the same choice of tube, we need to use set of four EL34/KT88 and set of four 12AU7, that will limited your tube rolling practise because matched quad set of high quality vacuum tube is not only expansive but also difficult to come by. On the other hand, the amplifier will be more powerful, sounds more accurate, but with less harmonic and intimacy. The question is, when you select a tube amplifier, do you wan it to be more accurate? or do you want it to have richer harmonic?
