NO! NO! NO! A lot of well intentioned suggestions have been made, that in many cases would be good advice, but which evidently have been made without looking at the specifics of the equipment that is involved.
1)It appears that your power amp has balanced outputs, meaning that both the + and - terminals have signals on them (as opposed to the - terminal being grounded). If so, you cannot connect the sub's negative speaker-level input to the power amp's negative output terminal, or you risk shorting that output signal to ground (if the sub and the amp both have 3-prong power plugs, and depending on their internal grounding configuration). Instead, the negative speaker-level input of the sub should be connected to a ground point on the amp.
A chassis screw is PROBABLY such a point. You can verify that by using a multimeter to check for continuity between a chassis screw and pin 1 on one of the XLR connectors, or else by asking Krell.
2)The RCA input on the sub you are considering is designed to only work with an input signal that has already been low pass filtered and summed to mono, such as an "LFE" signal. Your preamp does not provide that kind of output, and so your only choice would be to connect that sub to the power amp outputs (the + outputs and a ground point).
Also, Magfan has made an excellent point about the fact that you don't need heavy gauge high quality speaker wire to connect the sub to the amp.
Regards,
-- Al
