Off hand, I do not know although this is usually an acceptable, if suboptimal, way to connect a sub in such situations.  My suggestion would be to contact SVS as they have lots of experience and, likely, have dealt with this before.

First, run speaker wires direct to your speaker from the amplifier, but NOT to your subwoofer.

Second, run wires from one speaker's LF terminals to the subwoofer's LFE RCA (low-level) input, BUT with 10,000 ohm resistors in series (in-line) with each wire between the speaker and subwoofer. Be sure to insulate all connections with heatshrink sleeving so nothing will short out.

This will prevent the loading that is affecting your speaker system and get a usable input signal to your subwoofer.

This is a quick fix for the problem, but it is also true that the subwoofer's high-level input  is defective, or it wouldn't be loading down your amplifier like that.

The high-level subwoofer input is specified to have a 2000-ohm input impedance, which should have no effect whatsoever on your amplifier or speaker. If it is a new unit, I would demand that the seller replace it.