Era 300 equalizer set up

The EQ is standard Sonos, so nothing to do about it, unless there is a general update. Stereo pair LR balance for Era300s isn't exposed by the app,  don't know why. Home Assistant is able to adjust it, if you bother setting it up. 

Calling support and asking to have these added as options in a future release wouldn't be a bad idea. A forum mod may see this and forward it but it seems a call has better results.

The most obvious reason (IMO) for a lack of L/R balance control is that Era 300’s are Dolby Atmos speakers by design that happen to play stereo. Sonos Five’s on the other hand are actual two-channel Stereo speakers; with no Dolby Atmos capability, therefore the option for L/R balance.

Dolby Atmos has specific parameters that must be recognized by the speaker. Allowing user control over balance would most likely destroy (or at least severely alter) those parameters to accurately decode Dolby Atmos content. Stereo is basically straight forward two-channel audio; whereas Dolby Atmos has a lot of electronic trickery/magic happening under the hood.

The only way I can see giving the user L/R balance controls for Era 300’s would be to allow the user to switch between Dolby Atmos and Stereo modes (i.e. on/off).

With sources like Amazon stations that switch format with every track that would be aggravating. 

My two 300 Pairs aren't what I would call excellent Stereo speakers, they remind me of some of my older gear that was designed to be omnidirectional and room filling rather than providing precise a location for each instrument or voice. My kitchen 300 Pair sounds good and gives an image even when standing under one and 8 feet from the other. No need for a balance control to get sound from both. The sound is even better with Atmos. 

Balance is a post-render gain trim, not a change to the Atmos stream. Sonos already lets you adjust surround and sub levels on Atmos systems, and Atmos AVRs routinely have per-channel trims, so user level control is clearly compatible with Atmos. A Sonos staff reply quoted in the Era 300 threads says balance "made less sense" for this product. That's a design choice in the app, not a technical limit. The speakers do accept per-channel levels: Home Assistant exposes and sets L/R balance on an Era 300 pair. Outside surround duty, most of what people play on these is ordinary stereo, where the Atmos argument doesn't apply at all, so leaving balance out for a pair feels like slightly overdone app level hand-holding.

There are two different philosophies on this, and inexplicably, both are tenets of  “audiophile” thinking:

1. Strive for the purest, most transparent sound you can, whether it be via components or room analysis, and serve that up to the listener as is. Witness the boutique brand amplifiers that brag about not even having a tone knob, for example.  aka the “Our ears know best” philosophy.
2. Let the user transform or modify the sound to their hearts delight, whether by equalizer or DSP, no matter how bad it sounds to anyone else.  aka the “Your ears know best” philosophy.

Sonos has proven to be much more in the former camp than the latter, with minor concessions like the 3 band equalizer aside. 

I do get your point, but balance isn't a tone preference. It corrects for listening position, putting the image back where the mix intended, which is arguably a camp 1 goal. So withholding a simple L/R trim still seems a bit overdone, especially as it's available on other stereo pairs.

I can’t answer for Sonos, except to give their history, as above.  I will speculate in that “balance” in the case of Atmos capable Era-300s is far more complicated than balance of a stereo pair, so they may find it better to default to Trueplay for the Era-300.