I have Resolution audio Opus 21 for more than a year now. For 6 months I also lived with Mark Levinson no. 30.6 reference DAC. I have not heard 390S, but 30.6 is a good benchmark of the ML CD players sound.
Mark Levinson has strong, dark, stormy sound that is appealing from the first notes. It has good dynamics that is evenly spread all over the ferquency range (very rare quality). Another strong point for ML is excellent bulit quality. ML negatives: it has a very strange stereo picture. It is wide and high but the sound of instruments (and voice) seems to come from small points suspended in the air. A very strange effect (that may work with big speakers with vogue stereo picture). It is also very limited in the depth of the image - there is layering but it is more heard like height layering than in depth layering. Sound is, despite the good dynamics, very laid back. There is no microdynamic, the sound is very robust, present but with reduced `life`. This is pipe and slippers component.
On the other hand Opus 21 impress with it microdynamics and rhytmic interplay. It is one of the best PRAT players, sometimes it even seems that the instruments that had a good rhytmic quality or pace (guitar, drums etc) dominate over vocals and masses of violins. Sound is very smooth, not very extended in width but with beautiful feeling of depth that give the whole sound an etherical quality (like electrostatic speakers).
So in short ML is dark, stormy, with authorative presentation and laid back sound.
Opus is very light on his feet, airy, spacey, pacey and full of rhytm with good microdynamics, transparent and smooth but alive.
After living with both for 6 months I can not say which one is better. ML is much bettter built with more elaborated perifery (power supplies etc) but can sound more processed because of that (some call this laid back sound). Opus is much more transparent and has truer timbre. They are different and wil work differently in varoius systems. ML is more at home in big american systems with big speakers, Opus is better in horn systems, with electrostatics and in PRAT oriented systems (Naim and co.).
For me, Opus is a better player but this is based on my system (my opus is connected directly to ATC active monitors). While listening through my hedaphone system (Stax SR 007) I slightly prefered ML because of the more direct sound. Opus simply do better what is important to me in sound reproduction than ML. Please do also note that Opus is much more influenced by the quality of the electricity, ML has very good power supply - so take this into account when comparing. I am using Furutech schuko connectors and Eichman Express AC power cable which suits Opus very well.