I'll take a guess. Audio Research Ref 6.
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As a point of information, the Ref 75se (as well as other ARC Ref amps which only provide XLR inputs) will not work properly unless it and they are provided with balanced pairs of input signals. If the amp is provided with single-ended inputs, either via XLR connectors that have one of the two signal pins grounded or via RCA-to-XLR adapters, maximum power capability will be drastically reduced, and distortion will increase.
The only way I am aware of that a preamp having single-ended outputs (such as the deHavilland Mercury 3 mentioned above) can be made to work with this amp is by using a Jensen transformer, or something equivalent, to convert the single-ended signals into balanced signal pairs.