Various implementations. Depends on what you want / need and how much work you are willing to put into it.
(1) GSSL board only (no Turbo, no SSC) -- this is called "Vanilla GSSL" (there's that long thread).
(2) GSSL & 1xSSC (but no turbo) -- SC is mono, the SSC board alleviates mono buildup in the sidechain.
(3) GSSL & Turbo (but no SSC board) -- modes are "Aarhus" (audio mono'ed into 1x SC detector ) and "Oxford" (stereo into 2x detectors).
(4) GSSL & Turbo & 2x SSC -- switchable between the two modes, with both modes offering side chain filtering.
Variant (2) introduces SC filtering to 'fix' the mono buildup in the SC (+6dB) of the Vanilla, plus it gives additional features such as a side chain EXT IN and tilt-shift filtering (some call it 'thrust').
Variant (3) fixes the mono buildup in the SC (+6dB) of the Vanilla by adding a second detector, but it does without any filtering option. The argument is that the SSC filter board was introduced only as a 'fix'. Also it does without the 'thrust' thing.
Variant (4) has the two modes plus filtering in both modes. To do this requires two SSC boards and careful consideration of how it all hooks up (( not plug-and-play -- headache allert ! )).
With only one SSC board, there should be another option:
(5) GSSL & Turbo & 1x SSC only -- switchable between Aarhus mode with SSC filtering VERSUS Oxford mode without any filtering. Different hookup.
Depending on how many filter options you really want or need, there should be yet another option:
(6) GSSL & Turbo only -- later GSSL board revisions do the SC filtering not by adding a SSC board, but by changing only one cap sitting before the rectifier in the side chain. Should be possible to do that on both the GSSL board and the Turbo board, I'd think.
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Personally, I like the filtering options also in turbo mode (hence 2xSSC boards), but partially so because I ditched the EXT IN and instead added yet an extra bunch of filtering options including active filters. Interestingly though, after all the headaches, I also came to appreciate the plain Vanilla & SSC version more with its simple filtering of the mono'ed sidechain. Mono'ing the SC might sound like a fault, but it's also kind of a feature as it 'enhances' stereo so to speak -- cos can't compress what it can't see).
If I were to build another GSSL+, I'd probably go for variant (6) and put filters on a veroboard. Or I'd go variant (7), a GSSL & Turbo, and implement 2 x EXT SEND and RETURN to hook up an external stereo equaliser.
