Hegel just responded saying that the HT mode sets the Hegel pre-amp to a "fixed high level" volume of 85 (85% of the maximum volume of the H590) - which is very likely too high, when the ARC 5se is even set to 0 (100% attenuation).
I don't understand why the "fixed high level" of the H590 would be set so high - except that its designed to be used with a HT receiver or integrated amp, which power / volume may be much lower and the a volume level of 85 from the H590 spread over 3-7 speakers, might not be too much for them to handle.
In any case, it's beginning to appear that it may not be a good idea to use a tube pre-amp with the H590 via the HT mode - too much of a risk to my speakers.
I've already tried the combo by just adjusting the volume of both pre-amps to drive the speakers safely - using both pre-amps simultaneously. The sound was fine, but much better when I just used the H590 pre-amp alone.
Thanks again...