It's all good, but to me it's OK to live few days with unbalanced depleted signal. ARC preamps have very high line-stage gain so the volume won't be deficient at all till the other unit arrives.
This does not see a good suggestion to me! In addition to depleated power output, many ARC amplifiers also make a lot more distortion when driven single-ended. If one is trying to evaluate the amplifier it won't be possible!
Since a short-term suggestion was asked for, so far the best solution has been Al's, to get a set of Jensen transformers (which will do and excellent job of single-ended to balanced conversion without editorial) to drive the amplifier. This will work quite well and is far less expensive than replacing the preamp! In addition, the preamp right now is a known value and will allow for a proper evaluation of the amplifier.
If found acceptable, then I would indeed explore the possibility of updating/upgrading to a preamp with a balanced design.