I tried to calculate electrostatic driver unit requirements a while back (posts by jcx were super helpful in getting started and I am fortunate enough to work with some audio engineers who were very helpful in filling in the gaps) so this may help:
First you need to know your listening level. Honestly you shouldn't be listening at or beyond 85 dB for extended periods (the OSHA requirements are a really minimum guideline here) and relaxed and safe listening levels are between 75 to 80 dB.
The peak/RMS ratio of audio depends on exactly what you listen to. Classical music has the most demanding peaks (18 to 22 dB crests) whereas modern compressed hip hop/pop/rock can have peaks as low as 3 to 5 dB and pre loudness war music is somewhere in between around 10 to 14 dB peaks.
SR 009 specs:
Electrostatic capacitance: 110 pF including cable
Impedance: 145 kΩ including cable at 10 kHz (the impedance would be at tens of megaohms at low frequencies and tens of kiloohms at higher frequencies)
Sound pressure sensitivity: 101 dB / input 100 Vrms / 1 kHz (I would assume the sensitivity would vary across the frequency response)
Maximum sound pressure level: 118 dB / 400 Hz
The 353X has a max output voltage of 400 Vrms (roughly 1130 Vpp). I believe Stax actually measures these whereas I think the Vpp given for the third party amps is generally derived and not measured.
How much voltage needed for something higher or lower than the stated sensitivity spec can be calculated by: 20 Log 10 (V / Vref)
The 353X would probably be in the range of the 313/323S/717/727II in terms of how much current it outputs (
5.5 to 6.6 mA).
More current is needed to swing voltage at higher frequencies. Electrostatic transducers are pretty close (though cannot be exactly) pure capacitors so you can get pretty close to how much current is needed.
2 * pi * frequency * Vpp) / 1000000 = slew rate (in volts per microsecond)
slew rate * capacitance (F) = current needed (A)
You can do the calculations with your listening level and extra headroom required for peaks but given the 009 can do 101 dB at 1 kHz with 100 Vrms and has a pretty flat response (
measurements) and almost anything anyone listens to audio wise would require only a full power bandwidth of 3 to 5 kHz the 353X really seems to have enough to drive the 009 unless you start listening to really loud levels.
Now you could say it isn't enough to reach "real life" levels (I have read that classical music can have peaks that hit 120 dB live) but in that case the 009 transducer itself falls short of being able to replicate real life volumes. Plus the musicians in orchestras generally wear ear protection themselves anyway so just turn it down lol
The 353X was going to be my amp of choice and I still might grab one in the future. I ended up going with a T1S first because I wanted a nineties Stax amp and I had hopes/plans (now thankfully materializing) of grabbing an Omega. Though now I realize I need the darker bronze faceplate version to match the Omega...
Even if you want to get a different amp later it wont be much of a resale hit on the 353X you'll lose like $300 (as compared to the hits of selling one of the multithousand dollar amps)
I remember reading somewhere (here or on the other place) that the 353X is basically a lower powered 717 with some transistors removed and that apparently according to Stax the KGSS is a copy of the 717 (I have read here that the 717 is a licensed version of the KGSS with a different output stage as well though).