My Marantz doesn't seem to have a Direct mode that I can see.
I know my previous Denon did but not apparently its' subsidiary or partner or conglomerate or what have you does not
Yes it does.
Check your manual. For your SR6011, the english manual as a pdf, Direct & Pure Direct are on page 140. It's even in the table of contents.
The Pure button is at the bottom right of your remote, under Sound Modes.
and on page 283 of your manual is a chart named "Sound modes and channel output" which shows the channels and playback formats that are available in each sound mode.
I know the Marantz's can play DTS-HD/MA quadio BD's as 4 ch because mine does.
If you do want to use Dolby Surround for heights, leave it in Movie or Music mode using the remote buttons. Your screenshot shows it's in Music mode with the choices in the drop menu box. Using the Pure button, press it until Direct shows on your UI. When done listening, you can press Music or Movie mode to take you back to the one you want for normal listening.
What you may find is that the back surrounds may play through the rears instead of the side surrounds. This is a result in how the decoder handles a specific encode. Sometimes the 2 are swapped. With 5.1, it comes out right. With 4 channels only, sometimes it will play in the rears. It depends on how it was encoded, whether in a 5.1 container with no center/LFE OR as straight 4 channels in a 4 ch container. If the surrounds are playing in the rears, you can go into the setup menu and temporarily set the rear speakers to none; this will force the quad back channels to play in the side surround speakers.
But you should get 4 channels in Direct mode & you shouldn't have to change your Oppo to PCM. I don't and all my multichannel discs, whether 4.0/5.1 BD-A, 4.0/5.1 SACD, 5.1 DVD-A play with no upmixing in Direct mode. Unless there's a specific reason, I leave my Oppo player in Auto and I have 3 of them (105, 205, 203).
And Marantz isn't a subsidiary. Denon & Marantz are sister companies in the D+M Group. Both companies have shared hardware platforms but each company has a few unique features.