digital combos, help, EMM labs, MBL, Weiss, Metron

Why would you want a combo over a box unless you
had some specific reason?

it's not a combo!
it's one-box.

What about the new version of the Meridian 808?

You should add Zanden digital to your list of auditions. You might find it and Metronome 'different' sound than the first three in your list. I also found that Same make transport works well with same make DACs.

Weiss is a lot better with it's own transport than not

This is a good, if expensive, option. By the way, "Weiss is better with ITS transport"; it's= it is

I would also audition the Metronome etc, first. The sound from these two differs distinctly, so a good place to start.

Presumably, you are bound to choose among the systems you list? Or is there choice beyond that (Zanden, for example)?

Including gear from dCS your list is very good try to audition and if you go for combos stay in the same brand.
Although I had good experience from Esoteric transports and dCS dacs, but for example Weiss is a lot better with it's own transport than not.

Mostly you should thinking matching with the rest of your system.

First Scarlatti, after MBL,EMM labs, Metronome, and yes you can mix transports and DACs

Don't forget Theat Digital Gen8 DAC. They just upgraded it to a series II. Something to look at.