Subwoofer and preamp output impedance

when you use a splitter, that will be paralleled, so if the sub has a 10k input impedance your preamp will see about 9k Ohms overall. A huge difference from the 100k it saw before the sub was hooked up. The highs could be affected.

The input impedance on electronics tends to be purely resistive, so doesn't vary by frequency very much, which is different from speakers and headphones.

Unless the subs have very low impedances, around 10k or lower, you should not have any problems with 3 devices per channel.

One tip:  If your speakers are ported plug them when adding a sub.  It will lower speaker distortion and make it easier to integrate with the subs.

Thanks everyone. I went a different route. I bought a Minidsp Flex with its own sub outputs, so the issue has been resolved. The Minidsp also allows me to widen my sub search beyond models with integrated room correction. 

Excellent choice. That has a lot of adjustments.