The usual problem with SD channels is signal compression that leaves mosquito noise and block artifacts in the decoded image. The easiest, cheapest and most effective solution is to change service providers to one that delivers a clean signal. Barring that, the next best but rather expensive solution is to buy a product that includes mosquito noise and block artifact reduction. You can get this by moving up from the Onkyo 805 to the 905, which also gives you the Reon HQV video chip among its other great assets.
My third option is no longer available. It seems that Algolith has given up the consumer market and has dedicated itself to commercial broadcasting.