Thanks for the thoughts. It may well be that I cannot get the simple package that I have in my minds eye, but hey ho, finding something that sounds good is part of the funI think this is your situation in a nutshell. If you want to downscale towards a one box system, then I think you are going to have to accept that you won't get anything comparable with your old separates with valves.
The Naim had a reputation for being one of the best one box solutions out there, so if that doesn't cut it, I'm not sure what will.
You will be aware of similar offerings from other companies, but I'm not aware of any one box system that uses valves.
NAD and Lyngdorf options give you the bonus of DSP room correction that you are after, but they're both Class D, so won't sound like valves.
You may get some very high quality sound from a streamer plus actives as others have suggested, but will it be comparable with your old system?