Also wondering if the Sunrise/Horizont schematics can be modified similar to Valhalla 2 to be optimized for both high and low impedance headphones.
The amplifiers differ too much to compare.
In case of the Valhalla2 there are 2 output resistances to 'choose' from. 14 Ohms or 3.5 Ohms.
The reason there is some difference in output R has to do with the topology (the way the circuit is built) and the amount of feedback.
It's an 'all tube' design.
More feedback = lower gain = lower output R
Less feedback = higher gain = higher output R
Those aspects are directly coupled within the design.
It also means that even though the valhalla can be used with low impedance headphones it is more suited for higher impedance headphones.
For the SR/Horizon things differ.
Firstly this is a (low voltage) hybrid design (more akin to Lyr than Valhalla, EDIT: in output stage)
The gain and output Resistance can be set separately and are not coupled.
The internal gain of the amplifier is not determined by the amount of feedback (which it is in Valhalla)
In SR/Horizon the gain is determined by the tube used and there is NO overall feedback at all.
The input signal can be attenuated (so not gain control technically but attenuation control).
The amount of attenuation can be changed quite easily by the user (SR-III, Horizon III).
For the older versions the 2 gain steps are fixed.
The output R can be set in 3 steps: 1.5 Ohm, 35 Ohm and 68 Ohm for SR and 1.5, 35 Ohm and 120 Ohm for Horizon.
Both SR and Horizon can be used for low and high impedance headphones.
the SR can provide a bit more current in low-Z headphones while the Horizon can provide a higher voltage for higher Z headphones or low Z headphones that could best be driven from 120 Ohm.