My understanding is that if you're getting a signal of any quality from the HU but the actual data being sent is inferior, all the processing in the world isn't going to improve that. ie, signal processing improvement isn't going to change the quality of the data, it will just make it easier to read (clarity).
What I'm getting at is that you're better off with a great set of speakers with a good HU, than you are with crap speakers in any capacity, or a crappy HU with any level of post-HU signal processing. ie, an HU with a high quality DAC, preamps, and opamps going to great speakers is much preferred over a nasty bottom-of-the-line HU going to great speakers, no matter what else is stuck between them to try to improve the clarity of the data being sent from the HU since the quality of the data is still inferior.
Start the chain (the HU) with something of good quality and you won't have to spend extra to try to repair or fix what is being output by it.
Please correct me if I'm wrong.