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You may google "Sophia Electric 91-01 300B amplifier" for answers.
Here is a link for one:
I actually use Studer A727 to drive my 91-01 directly. A727 has variable output that can drive Sophia Electric 91-01 well enough. You may Check search DAC with volume control if you prefer computer output, or vintage Studer/Revox/Philips/marantz cd players with variable output for traditional CD discs.