So, of course, one cannot judge sound quality by specifications alone. The price of 180,000 yen = $1625 according to my currency converter. Nevertheless:
Max output: 550Vpp = 190 Vrms
Distortion: 0.1% at 100Vrms, 1 kHz
Freq response: 10-100 kHz at 50V output
Gain: 59 dB for e-stat headphones, 48 dB for speakers (note most speaker amps have 20-26 dB, 48 dB is 100x greater than usual), no spec for dynamic/planar headphones, which are usually around 10 dB)
By comparison, Stax 353x
Max output: 400 Vrms (roughly double the max output = 6 dB higher volume)
Distortion: 0.01% at 100Vrms, 1 kHz (solid state amps tend to have better distortion specs than vacuum tube amps)
Freq response: DC to 90 kHz into SR-L headphone (no output level given, but in the past Stax has always used 100VRMS output)
Gain: 60 dB
The KGSS has been superseded by other designs, but specs from KG's original article (copyright 2000):
Max output: 1200Vpp = 420 Vrms
Distortion 0.008% at 800Vpp (280 Vrms) from 20-20kHz
Freq response: DC-45kHz into Omega II, no output level specified.
Gain: 60 dB
So, in a nutshell, max output voltage is half, distortion is higher, gain is very similar.
According to the website, the output uses plate chokes (i.e. an inductor). Now, plate chokes have their lowest impedance at low frequencies, whereas Stax headphones resemble a capacitor and have their highest impedance at low frequencies. Music tends to have its maximum levels at low frequencies (below middle C). This means that most of the amplifier power will go to driving the plate chokes (i.e. wasted as heat) rather than driving the headphones, which is the whole point of the exercise.This not only wastes power, but increases distortion. By comparison, both the Stax and KG designs use constant current output loads, which allows the bulk of the output power to drive the headphones. The Woo WES used plate choke outputs with a more powerful output tube (EL34) than the 6V6 used in this amplifier, and had mixed reviews.
Finally, as a general rule, amps which can work with normal headphones, e-stat headphones and speakers are a compromise which are not optimum for any as the requirements for each type of transducer are so different.