Because the 300s have respectable bass I’d be looking at the Sub 3 or Sub 4.  Of course I like electronic music.

Thanks MoPac. I have been thinking about that too - whether the mini will have enough impact. As I say though, I don’t really want more bass, just more clarity through the mid and high range. I know a sub generally would permit this but wanted to see if anyone had experience of whether it makes a difference in that respect. Whether it is a mini or not, I assume the effect would be similar. Cheers. 

If no one offers first-hand experience, you could use the 30-day return period to try for yourself. Return them if you’re not happy with the sound. 

It’s hard to define ‘materially’, sound is so subjective to the user’s ears. What may seem as a material impact to one person may seem to be nothing to another.

Thanks all. I appreciate it’s quite a personal thing. Just interested in what others have experienced. Might try, and return if no good. 

Tried my new Era-300s in a 10x10 foot office, they sounded awesom.

Had a Sub gen 2 just sitting there so I tried it with them, love it and the improved low end.

My ears are old and busted so I can’t make reliable comments on the higher frequencies.

I just added a sub mini to my era300 pair. Honestly speaking, it did not give me a “waoo” or impress feeling compare from upgrading from single era 300 to era300 pair. So i tested with different type of genre music and movies. With lower volume, the overall improvement is like 10%? if you listen on a higher volume you can feel the improvement more. But in terms of mid and high it felt almost the same to me. It only will impress me when I go for actions movies and deeper bass music. But I believe still worth the upgrade because currently I’m using it as a stereo pair PC speakers for movies and music. It serves it purposes until I got my new TV and then arc ultra then sub gen 4. 

That’s really helpful Sean T. May I ask what size room they are in?

I’m in the living space with 6m x 4m x 2.4(H). I did more listening and movie yesterday, I would said it sounds better with sub mini(my wife agree as well). If I listen through Bluetooth by PC with youtube content (some higher quality content you can felt the speaker sounds better, some are not that why it felt “not impressive” to me). Comparing to amazon prime music(sonos app), or downloaded 4k movies(bluetooth-PC) will always sounds way better and more consistent compare to youtube(bluetooth-PC). Lastly, i know what I’m missing from the sub mini is, i used to have a ported sub(samsung soundbar & sub) and I love the rumbling base feeling when watching movies. That why at first I said it felts like 10-20% improvement. But after do much testing. I love my upgrades and you should too! 

You can see the layout below (purple hatch is the era 300 & sub location), not the best location but that where i use to watch movies and listening position. But is satisfy all my needs, and ready for my future upgrade (TV > arc ultra > Sub gen 4 > sell off sub mini). hahahahhaha

Thanks. That’s the same size as my room albeit layout is different. 
 

So basically, you reckon high quality streaming sounds better but it doesn’t really help the poorer stuff? I’m mainly interested in music and not movies. Thanks. 

I would said poorer quality content you need to go into the sonos apps to bump up or adjust the EQ and increase the volume so it sound better to my taste. Honestly, this is back to your personal preferences and is budget a concern as well. Example for me personally, I like music or movie with good bass(good quality). I would conclude for music itself the improvement with sub mini is like 20-30% better, but for movies is like 50%-70% better for overall experiences. But would recommend you to try it out with sub mini if you don’t like it I believe they have return policy. Because I tried listening to music or movies with good bass before(which i prefer), so to me era 300 pair the bass does not satisfy me “enough” so in the end I get a sub mini. Hope I answer your questions. 

Making comparisons harder, different quality streams may be from different masters/mixes so you aren't comparing just the speaker  but the mix.

Serious testing would be to pick an excelent hi-res track, make several lower quality copies and then compare them.

Same for comparing streaming, analog and Bluetooth reproduction, you need an identical source stream to get valid results.

I really appreciate the discussion. And I agree with everything that has been said. Budget isn’t a massive issue but if I wanted wireless hifi I’d have needed to pony up for LS50s plus a sub. So that’s £2.5k including stands. Very happy with the compromise at £700 for the Sonos. Just wondering whether they’d get closer to the Kefs if I introduced a sub. That’s really the question I guess. But I think I’m going to have to try it. TBH, I don’t expect the sub to elevate the 300s enough to be worth the £330 but the curiosity means I think I need to give it a try. There’s tons of bass. Too much really. I just want sharp mids. And that may not be obtainable at this price point. I just don’t really want to add something which makes the balance worse.