Hello Seamus,
the entire german (as well used at EMI and Decca/Teldec) EQs were all optimized for tone/frequency enhancement and not distortion as nearly everybody uses an EQ today.
Very selective designed and efficient, if you know how to used them, with your other equipment; with a minimalistic phasing problem.
There are different versions, made by several german companies all with reference to the IRT "Braunbuch" specification derived from the prewar german "Tontraeger Standard" = "Sound recording standard" developed for tape and records... After the war just minor modification regarding dynamic, frequency response were added.
The entire idea was to develop an tone alternating module, with a minimum of phase angle distortion, an entiere different concepet than today where everybody is so affraid of phase shift.
It is easy to deal with phase shift, than an distorted or offset phase angle is the basic line. Still hard to quantify there is not to much (unbiased) published info, so very difficult to put into perspective.
The entire german EQs, with some minor exceptions (like the ELA 150 stereo EQ) were based around LC filter, better described as resonance filter. Most famous Resonance EQs are the "Pulse Technologies" ones, or as every body knows them PulTech....
Basic concept is, use three different passive filters (for three different center frequencies), link them and buffer them via a tube or in the 295/395 case a transistor pre amp.
The three filters are set up around a center frequency, in most cased 60 Hz, 10kHz plus a mid/presence boost, so that all ranges overlay around 1kHz. Sounds more complex than it is and the end result sounds very pleasant.
There is an entire range you might want to try out, the passive Eckmiller, Danner and Klangfilm EQs, the buffered tube boxes and the later transistorized ones,
Siemens made
C72157-A24-A1 better known as W295a/b/c
C72157-A26-A1 or W294 the low pass filter
C72157-A24-A11 or W293 the high pass filter
TAB made (also sold as relabeled Telefunken)
same specs only with W3xx number. Some minor variations in used transistors, x-formers, etc.
Telefunken made several RC filter active and passive...
The later BFE or filtek, ANT (after Daimler closed Telefunkens Ela division, ANT was formed to service and supply replacement parts) are all op amp controlled, feedback EQs, that share nothing aside the designation, sound is very sterile.
If you want to buy and use the historic units, please be aware that no replacement switches are available (they break very easy) and the units rely heavily on the original Siemens/TFK and Valvo transistor for there great sound. There is no equivalent that works satisfactory...
Best regards,
Oliver