By the way, if you do go the route of adding just a low powered sub, I’d probably try EQ-ing some lows out of your main EQ (and lowering the low end of any multi-band master compressor) which will put less stress on your internal speakers (the low end is what chews up the power) and then just wind up the sub until it’s nice and full again. Maybe around 200-300Hz down, depending on how high up the sub’s range is…