For reference, there's an internal pic of your AVP here. It actually looks very similar to the low-end processors that Emotiva has offered over the last several years:
https://www.audiosciencereview.com/forum/index.php?threads/the-rise-of-audio-negociants.14384/Generally speaking, slimline and audio quality don't really go together, especially on processors. There's a reason for that. It takes space for a good transformer linear power supply as well as all the DAC and audio outboard board circuits. I cannot find a good pic of the 7703/7704/7705, but the Marantz is packed full of layers of circuit boards. The Anthem AVM50 is not quite as big, but it's still larger and full of boards.
Probably the best "slimline" type of processor I know is the Krell Foundation 4K, which is excellent, but now you're looking at $4.5k used. It's only 3.5" high!