I think what happens a lot is that people buy the No. 383 as it represent a good value for what it offers, but they're expecting more than what it's intended to deliver. This is not unlike the situation with the No. 390S CD player. If you want excellent performance (but not the best), it's a steal.
But if you want the best - and the best that the brand is able to deliver - you need separates. The devil is in the details, and a No. 390S does NOT sound as open as a No. 37 / No. 360S / No. 380S / No.
As my dealer would say: "What do you expect for $6,000?"
I myself seriously considered the unit - it's still quite a bargain, and I wasn't shooting for the best performance, just "very, very good". I had to pass on the No. 383 as my speaker choice (Aerial 7B) really needs more power than what the No. 383 delivers.
For the money (and this is just my opinion, of course), I think you would be better off with a used No. 300 series amp and a used No. 36/No. 38 combo. Yes, the DAC and pre are quite a bit older, but they perform much better, and they can both be completely upgraded to a 360S/380S, respectively, and you'll get a new five-year warranty when you do so. The prices I've been quoted for upgrades still makes them cheaper than new units, and with a new warranty, there's no investment to lose.
As for the complaints I've seen regarding the sound, the Mark Levinson brand is known for sounding like there's nothing there. I've noticed that a lot of owners don't even take advantage of its fully balanced operation.
Personally, I would probably be disappointed if I chose a very accurate loudspeaker rather than my beloved and very musical Aerials.