Series I = superior electronics!

GML 8200 Two Channel, 5 Band Parametric Equalizer Brought to you from the man who invented the term “Parametric Equalization”, the 8200 has been an industry standard for over twenty years and can be found on virtually every major recording studio’s stereo bus. Each of the five broadly-overlapping bands offers 15dB of Boost or Cut and adjustable bandwidth (or “Q”) from 0.4 to 4. The lowest and highest bands also can be switched to Shelf mode. Quite simply, the 8200 is the archetype Stereo Parametric Equalizer. Its extraordinary resolution, benchmark transparency, generous headroom, and surgical precision have been the reference for many other equalizers, but exceeded by no other. The 2U 8200 includes one 8355 external power supply.

With astonishing precision and sonic accuracy, the GML 8200 Parametric Equalizer has been an industry standard for over two decades. Its operational characteristics have been honed through many years of use in the most critical recording and mixing situations and can be found on virtually every major recording studio’s stereo bus. Each of the five broadly-overlapping bands offers 15dB of Boost or Cut and adjustable bandwidth (or “Q”) from 0.4 to 4. The lowest and highest bands also can be switched to Shelf mode. Quite simply, the 8200 is the archetype Stereo Parametric Equalizer. Its extraordinary resolution, benchmark transparency, generous headroom, and surgical precision have been the reference for many other equalizers but exceeded by no other.

GML has this to say about Series I vs Series II:

Series I units can be easily distinguished by their paddle-actuator toggle In/Out switches, in contrast to the illuminated pushbuttons of the Series II.  Aside from minor cosmetic details such as this, there are more significant variations which could account for any perceived differences. The Series I employed separate in/out buffer cards for each channel, each with its own local regulation and power filtering.