@PatrickB123 

Each Sonos room is a certain configuration of device(es). A single speaker, a stereo setup or a surround setup. You can bond a sub to ONE configuration, not to more than one. 
You could buy a second Era300 and configure a 7.1.4 surround setup with Beam, Sub Mini and 2x Era300 which you can use for tv and for music. 

Hi Ralf. Thanks for your answer. 

I've also found that when you 'Remove sub' from the Era300 there's an option to reassign it to a different speaker, in this case the Beam. Works vice versa too and only takes a few moments. Happy with this solution for now.

Hi Patrick,

of course any working solution is ok, if you are happy with that. Just want to add that it’s not the recommended way of using and for example you will have to run a trueplay setup again each time. 

Hi Ralf. 

I take your point, but it's interesting to find out that essentially you can either set up your Sonos system for music or TV but not both. I've invested in a brilliant music speaker - the Era300 and an excellent TV sound bar - the Beam. I've added a sub for a bit of extra bass. A single Era 300 doesn't really integrate with the Beam - fair enough, but to have to choose whether to use the sub for music or TV seems restrictive. I'm new to Sonos and love the quality, but the lack of easy music / TV switching has come as a bit of a surprise. 

In a “traditional” wired amp-and-speakers system you’d have to go through the equivalent to move a sub from one system hooked up to the tv, to another set up elsewhere. How did you think it could be done?

Hi nic9669a. Thanks for your input here. 

To answer your question, the way I thought it could be done was to be able to choose exactly what speaker or combination of speakers to play a source through. TV, music, line in, etc. So to have a speaker group for music - Era300 and sub, then a group for TV - Beam and sub. 

Just interesting to find out that's not how it works. 

@PatrickB123 

That is a result of the difference between a „group“ and a „configuration“ (in Sonos term called room). A room / configuration is a permanent setup like a stereo pair or surround setup that even persists after restarting devices. A group is a more temporary setup of synced rooms that doesn’t persists after a restart. 
And as a single Sonos sub on its own isn’t a room it can’t be grouped in a flexible way. It always has to be bonded as a permanent part of ONE room / configuration. That’s the reason why you can’t use it in such a flexible way. 

See what combination obtains the most bang for the buck for you and permanently use the Sub in that combination. I would think that the 300 could do without the Sub for music, while the Sub will do effects for movies more than the Beam alone can; easy enough for you to decide what works better for you after some easily done playing around.

Sub also does best for each mode in specific locations related to where the other unit is placed, so even if you could do what you would like to do, it may not yield optimal results without moving the Sub around each time you switch it over.