If you're looking to upgrade from your PS Audio DMP and Directstream MK1 to a single box solution, there are a few options to consider. The Accuphase DP-720 or 750 and the Marantz SA-10 are both solid choices with excellent sound quality.
The Accuphase units are known for their high build quality and reliability, although they may be more expensive in the USA. The Marantz SA-10 is also a well-regarded unit with excellent sound quality and a more affordable price point.
Ultimately, the decision between upgrading to a single box solution or sticking with the PS Audio gear and upgrading over time will depend on your personal preferences and budget. If you're happy with the sound of your PS Audio gear and appreciate the ease of upgrading over time, then sticking with the PS Audio upgrade path may be the better option. However, if you're looking for a single box solution with superior sound quality, then the Accuphase or Marantz units could be the way to go.