The APA150's volume does not control the RCA out. It was one of the small amps I considered, but several reviewers on Amazon, Parts Express, etc. noted that the RCA out was not controlled by the volume knob and so it couldn't be used for a sub in that way.
If I decided I wanted more power than the TP20, I'd probably consider the Topping TP60, the APA150, and the Emotiva a100, but all are are at least twice the price (and twice the size) of the TP20, so I'm going to stick with it as long as I can. Plus, for any of these, I'd still be using the hi inputs on whatever sub I buy.
Speaking of which, any further recommendations of <$150 subwoofers for a small home office, either from the ones I listed in the original post or otherwise?
Thank, again, for the help you all have offered. It's been very informative and cleared up some lingering questions.