It really depends. I’ve run the gamut from having completely dedicated two channel and HT rooms with absorption and/or diffusion everywhere, to just finding the best spots for speakers and running some light diffusion. In my experience, and of course this is different for different people perhaps, it ends up sounding better with excellent speaker placement and very little to no treatments. Absorption in particular for me always takes away from the liveliness of the sound. Lately light diffusion via shelves on either side of the room, and objects on those shelves to break up the first and second other side reflection points has given me the best sound. Some light EQ can take care of the most pernicious room modes, which wall hanging absorption can’t fix anyway. As long as you’re starting with decent room wall lengths (that don’t multiply room mode strength by being too similar in size) in a decent size room, you can get away with very light acoustic treatment both physical and electrical.
Still setting up my McIntosh gear at the new place for the Susvara, but it’s close to being finished. I really like the MHA150 and MCT80 combo, as well as the Auralic Vega for music via PC. I can listen to almost everything in my collection except DVD-A and vinyl, but I have those covered on the speaker setup.