Talking to subwoofer manufacturers can be challenging.

If I use an audio RCA out on a preamp to connect to a subwoofer, which input on the sub do I connect the RCA cable into? I don't think I can use a high level input because my amp is fully balanced.

When connecting a subwoofer to a preamp using the RCA output, you need to use the line-level input on the subwoofer. Most subwoofers have both RCA inputs and high-level inputs, but in your case, you should focus on the RCA connection. If your subwoofer has two RCA inputs for left (L) and right (R) channels, simply connect the RCA cable from the preamp to one of these inputs, typically the LFE input (often labeled as L). If the subwoofer only has a single RCA input for LFE, you can use that to connect to the preamp. After connecting, you’ll need to adjust the crossover frequency and volume on the subwoofer to blend smoothly with your main speakers. Since your amplifier is fully balanced, the high-level input is not appropriate, so using the RCA input ensures optimal sound quality and prevents potential signal interference.