22!!! Listen, the stock system sounds like crap at that level. It would be audibly clipping so there is no point in getting a fill sub to perform at that level with the stock sound system. And yes, I have the BO 14 speaker tech pkg. I would never run it over 15...it sounds like complete crap otherwise. Honestly, I never even knew the stock system went up to 22... crap, I say.
Also, since you are taking speaker level inputs, when the amp that feeds the stock "subwoofer" starts to clip, there goes your signal. I would imagine the whole system limits out well below a setting of 20. I personally target a max sound level of "15". Anything more, and you are better off redoing the entire system.