You can hook up any preampflifier you like to the 3,5mm stereo aux to usb-C adapter of your of your Beosound Theatre and then connect all desired analog (or digital, depending on the preamp's connections) sources. 2 random examples: Yamaha AVS 700 https://spencertified.com/products/yamaha-avs-700-natural-sound-av-selector Sony TA-E9000ES: https://audio-database.com/SONY-ESPRIT/amp/ta-e9000es.html  

Is that the elegant B&O-style solution that you would like?  

These where just 2 examples and one out of many solutions for your problem. If it's elegant is a matter of personal taste. I'm using a TA-E9000ES on one of the Beosystems as many members of the family don't think "A.Mem" or "V.Mem" are very intuitive names when they want to listen to music. When a preamp has a display and can show "Blueooth", "Chromecast" (or any other free text) on its display it's helpful. You can hook up any preamp and highend preamp you like to your system and hide it in a cabinet. Don't worry: Some of those devices are much more highend than Bang & Olufsen (qality and technology). There are preamps etc. for custom installations, too and those can be controlled through professional home automation solutions (RTI, Crestron, KNX ...)

Thanks T.W.G I now think you are right.  I will have to find a suitable preamp to handle my needs as I don't think any other solution will become available soon. I have a facination with old technology.  I have working telepbones from the 1930's in my home and I love old audio equipment, including an AM radio from 1924.  My main system is built around the Beosound Theatre and I would love to be able to show off old sources to people who aren't familiar with them anymore.  I guess this is a very niche market today.    

That sounds like a very nice hobby and I can share the fascination :-) Than a good preamp that suits your number of input sources should be a good way. If you buy a unit with a display you may improve comfort with custom source names. Have a look in the user manual or ask the seller if the preamp allows to rename input sources to show them accordingly in its display. For example "AM Radio" instead of "Tape".

I have the same interests and I love getting old technology to work with new. Some time ago I have a Boscom source switcher that allowed an extra 3 inputs and was controlled by a Beo4 . I found this link to it, but cant paste it for some reason - google search Boscom AAUX Expander - not sure if you can buy it as an individual or if you have to be a supplier or how much it is    

Here's the link: 

Location: Warwickshire, UK

My B&O Icons:

BeoCenter-9500.png

Beoremote-Beo4.png

Beovision-10.png

Beotime.png

Beosound.png

Beosound-Explore.png

Beosound-Essence.png

Beoplay-P2.png

Beoplay-M3.png

Beoplay-H4.png

Beoplay-H3.png

Beoplay-Form-2i.png

Beoplay-Earset.png

Beoplay-A2.png

On Guy said

Here’s the link: https://dk.bosscom.com/products/details?id=1143

Interesting device. The manual does not seem to be available at the page, so I am wondering which remote buttons are used to select the inputs to not interfere with other sources on the Beosystem that it is used with. For BS Ouverture it could make sense with Tape2 and Phono as the Ouverture automatically redirect those commands to it's AUX connection

On Guy said

Here’s the link: https://dk.bosscom.com/products/details?id=1143

Interesting device. The manual does not seem to be available at the page, so I am wondering which remote buttons are used to select the inputs to not interfere with other sources on the Beosystem that it is used with. For BS Ouverture it could make sense with Tape2 and Phono as the Ouverture automatically redirect those commands to it’s AUX connection It looks like it might be a Oneremote product. I found it looking for something else in their list of manual  Login to view image