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The X350.5 has specified input impedances of 30K unbalanced and 20K balanced, as indicated under "Technical Specifications" here, although I suspect those numbers have been mistakenly interchanged. As with many ARC line stages and preamps ARC recommends a minimum load of 20K for the Ref 5se, although it isn’t made clear if that applies to the balanced outputs or the unbalanced outputs or both. The reason 20K or more is recommended is presumably because the output impedance of the Ref 5se and many other ARC line stages and preamps rises to considerably higher values at deep bass frequencies than its specified value, which is presumably based on a mid frequency such as 1 kHz.
Those numbers suggest that impedance compatibility between the two components would be marginal at best. However Stereophile’s measurements of the Ref 5se’s output impedance were as follows:
At high and middle frequencies the Ref 5’s output impedance was slightly higher than specified, at 628 rather than 600 ohms balanced and 322 rather than 300 ohms unbalanced. However, at 20Hz the impedance rose to 1447 ohms balanced and 637 ohms unbalanced, which, with an extremely low load impedance of 600 ohms, rolls off the low bass by 3dB at 15Hz.(Although that paragraph refers to the "Ref 5" the review and the measurements were of the Ref 5se).
1447 ohms seems comfortable relative to a load impedance of 20K or more.