As far as I know Pass amps can drive anything... Not as familiar with Threshold, but after 2 minutes of web reading it seems they also focus on similar goals as Pass and can deliver power to super-low Ohm loads. I would be shocked if the Pass 350.5 or better can't drive anything to super-high levels and/or blow 70% of speakers altogether with a high-gain pre-amp if they have no protection circuitry.
It would actually be fun to take some X1000.5s and see how many speakers (yes, even super-high end) can't be destroyed with it. I suspect very few would survive, including 86 db sensitivity speakers.