Royal Navy
100+ Head-Fier
I have used my speaker amps without an adapter. As have plenty others including @GoldenOne.
Accuphase solid state to Hifiman planar drive should be fine without any impedance issues. I have checked with Accuphase as well.
The 170W output power is huge so you definitely want to be careful with the volume knob. If you unleash the full power, you will most certainly damage the Susvara, not to mention your ears. One of my friends fried his Susvara this way. He was using a headphone amp that he had maxed the volume on for a Susvara. Later he used it as a pre-amp to a power amp, but forgot he had the volume maxed.
As for good adapters, in terms of quality, try Brian Fox at zynsonix: zynsonix@gmail.com
https://www.zynsonix.com/
I'm so sorry to hear about your friend. Hope his hearing is not damaged.
May I ask the position of the volume knob when you run the Susvara directly with the E380? Like does it pass over 9-10am? Thanks!