I would expect that the input impedance of each of the amp’s inputs is the same regardless of whether the amp is being used in mono or stereo.
The 10x rule of thumb guideline for impedance compatibility should ideally be applied at the frequency for which the output impedance of the component providing the signal is highest. Most impedances are specified at a mid-range frequency such as 1 kHz. It is very common for tube preamps to have output impedances at deep bass frequencies that are much higher than that specified value, often 2K or 3K or even 4K ohms. That rise at low frequencies results from the output coupling capacitor that is used in the majority of tube preamps. In those cases a significant degree of deep bass rolloff may result if the load impedance is less than 10x that higher value, depending on how much difference there is between that low frequency output impedance and the output impedance at higher frequencies.
If as is often the case the component’s output impedance at 20 Hz is not known, and is not indicated in published measurements (such as Stereophile often provides), then to be safe a considerably higher ratio than 10x should be used, perhaps even 50x or 75x IMO. Especially if the component is tube-based.
In any event, to assure optimal results I would consider ARC’s 20K recommendation to supersede any such guidelines. The question, though, is whether or not that figure applies to the balanced outputs as well as the single-ended outputs. As I indicated I would have some concern that the corresponding figure for the balanced outputs might be 40K (i.e., 20K per leg). You might want to pose that question to ARC.
If the Spectron has a balanced input impedance of 50K, given that the LS-17SE’s two XLR output jacks (per channel) are most likely driven by the same output stage (and are probably just wired directly together inside the rear panel), driving the two inputs of each monoblock from those two jacks (or from one of the jacks using your existing splitter) will result in a load impedance seen by the LS-17SE of 25K.
Regards,
-- Al
