On the topic, I can finally share some data from testing my Lundahl LL1630 setup and limited testing with all the gear I have available... It's fascinating to see it all plotted out and compare STAX gear to transformer setup. The STAX SRM-1 surprised me with its low end performance but also how quickly it falls off, also interesting was its behaviour changing quite dramatically based on the load used. Even a 2 wire reading from my oscilloscope (1mOhm) would drag down its output significantly...
In contrast the Lundahl LL1630 just doesn't give a crap really, it will deliver consistent results regardless of what load I put on it and is basically flawless across the FR (voltage output), its only limitation seems to be its fall off at 20hz and lower (270Vrms limit when the core saturates) it may be possible to work around this with a different winding configuration or possibly series them???
Out of the amplifiers used, the Topping LA90 was the most linear, clean and consistent from what I saw and 2 of them fit nicely on a desk with balanced inputs used. Also threw in some random testing with different amplifiers to see how it impacts the response and also a 1960s Encel tube amplifier for the LOLs of "tube sound"
Tried various loads from 120pF capacitor to 150kOhm resistor with capacitor in series / parallel and ended up settling on the 120pF capacitor alone with a single wire reading off my scope and multimeter (10mOhm) to give the SRM-1 a fair chance. I chose 150Vrms as the reference based on my listening levels.
What was used:
STAX SRM-1 mk2 (Fully restored and calibrated)
STAX SRD-7 modified with 4.7ohm resistors and switching removed (performed the best compared to my other SRDs)
Lundahl LL1630 - Driven by Topping LA90 Monoblocks (Roughly 90Wpc)
Lundahl LL1630 - Driven by Sony TA-N9000ES (Roughly 200Wpc)
Lundahl LL1630 - Driven by Luxman LV103 Tube / MOSFET Hybrid (Roughly 75Wpc)
Lundahl LL1630 - Driven by Encel CSM-40 OT Tube (Roughly 18Wpc)