Hi Dave,
Thanks for providing the additional info. You've got some esoteric and unconventional equipment there indeed!
As a consequence of that I would suggest caution in extrapolating from the experiences of others who may also be using low powered amps with long speaker cables. As you realize, your amplifier is a very different animal than the SET amps or other low powered tube amps that are more commonly used with high efficiency speakers. In contrast to those amps, yours clearly has a very low output impedance/high damping factor, as evidenced in part by its ability to double power as the load impedance is halved from 8 ohms to 4 ohms, and to apparently (based on the description) be able to handle 2 ohms with reasonable comfort.
That said, I agree with Zman's comment that you should be fine with the right cable. Choosing the right cable may be somewhat hit or miss, though, because there probably isn't much of a base of other users to draw upon who have relevant experience, given the presumably limited number of people using similar equipment in conjunction with long runs. So return privileges, or at least the ability to re-sell with minimal loss, are particularly called for.
I second the suggestions of low capacitance (which will help assure that the amplifier is happy) and shielding. I would avoid ultra-low capacitance, though, because that commonly (although not always) goes hand-in-hand with high inductance, which you also want to avoid for a long run, as high inductance (which is proportional to length) could affect transient response and tonality in the upper treble. The NAC A5 cable you mentioned is an example of a cable having ultra-low capacitance but somewhat highish inductance, per the specs listed in post no. 2
here, and I would avoid it for a long run such as this. Its resistance also appears to be uncomfortably high for a long run, corresponding to about 16 gauge. I would not use a gauge narrower than 13.
Clio09's suggestion of the
shielded Supra cables looks intriguing. The Linc 2.5 corresponds to about 13 gauge, and the other two models to about 12 gauge, based on the resistance specs shown
here. Inductance seems reasonably but not exceptionally low (based on what seems to be the safe assumption that "mH/m" means microHenries per meter, not milliHenries per meter). Capacitance isn't specified. I suspect that it is within reason, but you might want to contact them and ask. Anything significantly greater than 150 pf per foot would be grounds for rejection, IMO, given the uncertainty about the amplifier characteristics, with a significantly lower number being preferable.
Regards,
-- Al