Yes,
They have anything form 250 to 500 volts charge on the electrostatic elements. This, depending upon which model, is derived from, in my case, a SRD - 5 energizer. It is very, very low current so there is no risk. The SRD - 5 is powered from the mains. The left and right loudspeaker outputs from your A21, or in my case my 303, 495 or 909 connect into the SRD - 5, into which the headphones also plug.
It is difficult to make a subjective comparison with any loudspeakers, except perhaps the original Quad Electrostatics which have "Full - Range" elements, just as the Stax have. This means no cross-over networks and thus extremely fine "Phase - Response", right across the whole audio spectrum. Indeed Quad's ELS were used and perhaps still are, as a reference by conventional loudspeaker designers/manufacturers.
My Spendors have just been fitted with new original drivers and their performance is superb at modest power levels. Yet, when compared to my SR3 Stax headphones, the only advantage they have is, one is not tied down by a headphone lead.
In addition, my Sugden Headmaster was bought for the very same reason you intend - we will be downsizing to a smaller property. The Headmaster is a Pure Class A Headphone/Pre-amp and having had an original Sugden decades ago, I knew just how good they are. However, as yet I do not have a pair of headphones to plug into the Suggie so, I dug out my SR3's
They were brilliant driven by my 303 and 405. However, I wondered if I could "beat the system" by using the 405 at low levels to keep it operating in Class A, as the Sugden does. Sadly, there wasn't sufficient power, but that provoked me into speculating the 909 would operate in Class A to a higher level.
It is all very fortunate for me as most of what has gone on was totally unplanned. The net result is a brace of amplifiers, the Suggie and the 909, with extremely low noise; of particular importance when playing music at very low levels, an almost unrivalled dynamic range and an "Attack" which is swifter than a rifle shot (figuratively speaking).
The driver behind all this is my obsession with Class A, without the means to afford a top of the range Sugden. The Quad 909 however, makes a pretty good substitute - if not quite the real deal.
As for your Sugden, were it mine - I would not part with it, "For All The Tea In China".