hello,
the rule of thumb about impedance matching for TUBE amps, is you should try to match the amp and transducer impedance to have optimal power transfer .
tubes being voltage amplification devices, they tend to be more powerful with higher impedances loads ( more Volts into more ohms for the same power )
Solid state devices are the other way around, they are Current amplification devices so they tend to be more efficient with lower impedance loads ( more amps into less Ohms, therefore less volts for the same power )
this is especially true for dynamic headphones, as they usually dont have a flat impedance curve, so the output impedance of the amp has it's importance
for planars, they are usually a straight line in impedance, so they shouldn't be affected as much sound wise by the output impedance of the amp.
If i remember correctly the output impedance are :
Low : 32-64ohm
mid : 64-300ohm
high: 150-600ohm
I would try the medium setting with your susvara. maybe the high Z is less controlled so the bass is bloomier ?
I wouldn't worry too much about it for you planar, it shouldn't have a big impact.
do you use the susvara balanced or single ended ?
with my aryas I use Balanced and Low impedance mode. If I go to High Z mode, it gets a bit louder, so my first impression is to think it is better, but if go back to low Z and give it 1 click more on the pot, loudness is the same again.
YMMV
On the other hand, I will take the example of my 80ohms utopia cans.
A lot of reviewers say they are thin in the bass... but they only ever listened to them on solid state amps with low output impedance.
when you look for the impedance graph :
you can see a big jump to 300 ohms in the bass region.
So with a tube amp that can put out a lot of voltage, it means you have more power coming to the drivers at those frequencies.
Therefore the utopias sound a lot fuller.
I use them on the medium setting, and the difference with a SS amp is quite something ! now they are engaging !
High Z sounds great as well but I get too much hiss so I stick to mid.
Low Z sounds more like SS
here is your susvara for comparison
Seeing this graph, I would go mid Z, KT88, balanced out for max power.