Hello Geoff,
there are quite a lot of different stories and myth out there, I always get contact if I could build one from scratch, etc. But fact is you spend more money than what the end result would be worth. Over the years I rebuild several consoles, one EMI Cologne one... As they come they are more museum pieces..
The EMI/REDD console concept was mainly based on the IRT/Telefunken/Siemens V75 series. Closely examined the REDD37 is a low budged version of the Telefunken/Siemens Stereo consoles. Attached is a standard Telefunken Stereo console of the same time compared to the EMI 4 channel this 12 channel is quite the Lincoln of its time.
I will attach a few images, like routing, etc. so you can see the general idea. EMI in Hayes worked closely with EMI Cologne better know as Carl Lindstroem. They build their consoles on the same Danner chassis than Telefunken/Siemens, but replaced several high price units like routing, buffering and faders, etc. with self made ones, even than the console was close to $200000 in the late 1950s...
Some of their in house made units, like the buffer known as Reed47 came right out the Siemens circuitry tube hand book, they used mostly Malotki and Haufe x-fomers. Others modules like the EQs were quite incredible in design, layout and sound.
The entire console was modular and could be modified and updated/upgraded without any problems, what was done frequently over the years....
The early units were build around Malotki V71 and Siemens V72S units, later upgraded with TAB V76S units that were modified to have a 15kHz cut... Aside the Danner frame and multi connectors, Siemens, Maihak and TAB amplifier, Malotki x-fomer, etc there were also the Painton faders, Eckmiller EQs and lots of Tuchel connectors...
The entire console was set up on the European standard 200 Ohm impedance platform voltage coupling vs. the standard American power coupling, the pan pot was a 400 Ohm.
Each of the input channels was set up, attenuator, passive bass cut into V72S (later V76S), than to an insert point, passive tone control, either EMI made or Eckmiller, Painton fader, optional T plug pad, into a splitter and pan pot set up.
From there the signal went into the four main channel,
V76S, fader and optional plug in/insert or attenuator than via track selector into the EMI made Redd47 buffer/line amp to tape...
Hope that answers some questions....