This is not ideal because the bass would lag behind the main channels. A/D conversion, D/A conversion, and processing cause delay, therefore if it's done to the sub, it should also be done to the main channels. That way, the signals stay in alignment, or even better, can be time-aligned according to the speaker distance settings in the setup menu. All filters, analog or digital, add noise and cause delay, and on technical grounds, digital ones have better performance (opinions to the contrary always seem to have been created before anything that was designed properly - or anything at all - was actually tried).