Ok, concerning Series One, the preamps are a little flawed imo, full gain with
attenuator in front, still sounds nice but noisy, the eq is sounding sweet but has fixed frequency
points not very flexible, could be made switchable for 2-3 freq. points?
There are two versions of Series Two :, one with opamps and one discrete, which is
close to Serie One, but has much quiter preamps, the eq points and circuit is also
a little different, but also fixed frequencies
At least two different types of transformers on Series One, don´t know about Two...
Runs at 40V, but can be used at 48V (comfortably) with the chunky transistors used.
2sa970 and 2sc2240 are good replacement types
For 500 style module one should think about how to implement balanced i/o.