There has been some talk of Mark Levinson (the company) developing a universal player. I believe this is more likely than another upgrade for the old ML 39 platform, even though I don't perceive any insurmountable impediments posed by the tray/transport mechanism for the 390s. A universal player is likely to be a completely new product versus a "ML 3900sx" extension with an upgrade path from the existing 390s.
Perhaps history will remember the 390s as one of the last one-box CD players that could present redbook material competitively against the new formats. Sort of the state of the art at the end of a product cycle. I bought a Sony VCR a couple years ago, the SLV 1000, realizing that the manufacturers were devoting all their energies to improving DVD players and that subsequent new VCR models would be progressively cheaper and less durable, capable, innovative, etc. The SLV 1000 was the best I could get for my "last" VCR. Let us know what you decide. Good luck.