Unfortunately, the Schiit website doesn't mention without a lot of digging that the Vidar 2 has floating grounds for its speaker terminals...which is what more and more class d and differentially balanced class a and class a/b amps have. Therefore, connecting a sub the way you are doing it is a problem and likely the reason that the amp is tripping.
This is from the Rythmik site:
Can I connect the speaker output from my digital power amplifier to the speaker input of the sub??
Our speaker inputs aredesigned to accept regular single ended power amplifier outputs where the black terminals are the ground. Most digital power amplifiers have complementary outputs. That is, the black terminal is not true ground. This can cause problem problem even when the user accidentally short the black terminals of left and right channels together. Using outputs from complementary output amplifiers not only can damage our amplifier, it can use damage the source power amplifier itself. Please confirm with manufacturer of your digital power amplifier before using speaker level inputs.
To be clear, the Vidar 2 is not a digital amplifier but it has the same floating ground (black terminals not being true ground) configuration as mentioned.
If you had a REL, they have a work around that I don't think will work for the Rythmik...therefore, the only connection you can make is line level from the preamp output to the subwoofer line level input.