In order to match the pre-amp correctly you need the SQD-2000 (the early version of the SQD-2010 & SQD-2020) - it has same matching smooth knobs - NOT scalloped like the later models.....
SQD-2000 = smooth knobs, discrete transistors mounted on 2 main single-sided (non-pth) circuit boards - each about A4 in size (on top of each other).
SQD-2010 = scalloped knobs, custom ICs mounted on 1 main single-sided (non-pth) circuit board - A4 size
SQD-2020 = scalloped knobs, discrete transistors mounted on 1 main double-sided (non-pth, using rivets to join one side to other) circuit board - A4 size.
SQE-2000 = smooth knobs
All 4 have world voltage selectors fitted - remove little rectangular cover at the rear....
Hope this helps for any future questions
Stephen in Auckland, NZ