Replace my Marantz AV7702mkII?

For example, does the 2 chan pre need to be on when watching a movie? I think so, but that means a tube pre is kinda out of the question, since it would burn through the tubes much quicker.

Your amp(s) will connect to the preamp.  The signal is routed from the preamp to your HT processor where the amp input, bypassing the preamp.  Some preamps need to be powered on, others don't. 

I use a tube preamp this way.  I listen to music much more than I listen to movies.  I don't worry much about tube life.  If I wasn't watching a movie, I'd probably be listening to music.  I don't generally use the front two speakers when listening to TV, I leave the preamp off and listen through the center channel from the HT processor.

does the 2 chan pre need to be on when watching a movie? I think so, but that means a tube pre is kinda out of the question

Well, yeah, but you don’t have a tube option at all using only a prepro, right? The point is, even a good solid state stereo preamp will likely provide better performance — and give you a ton more potential options — than using the prepro for stereo and would also allow you to just keep using your Marantz for HT duties.

If you want to use a tubed stereo pre there are a few that don’t require to be powered on for HT (maybe VAC, Backert, Rogue?) but not many. Another way to look at it is that if you find a tube pre you really like the tubes in many tube pres can last thousands of hours and aren’t necessarily too expensive to replace so may still be a worthwhile option for the benefit enjoying some tube magic.

For hookup you only only need to run the front L/R pre outs from the Marantz into the stereo pre and connect the stereo preamp to the amp powering your front L/R speakers and you’re pretty much done other than plugging your 2-channel source into the stereo preamp. Only requires an extra pair of interconnects. Hope this helps, and best of luck.

@soix Thank you for the information, it has been very helpful.