The gen 3 sub has improved hardware, but no additional functionality.    If your current sub isn’t performing well doing wireless issues or something of that nature, getting the gen 3 sub might help.  Other than that, there’s no real reason to upgrade your sub, IMO.  If there are new features introduced later on that interest you, you can just upgrade at that time.

To add to the above, it has improved hardware, but all hardware related to sound quality is unchanged - so no difference in that aspect.

I am facing the same question. The Gen 2’s are now going for $599. Is the Gen 3 worth the extra $100? Can’t say that I like the fact that Sonos has taken away the ability play music on an iOS device on older products either directly or through Air Play 2. I worry that if I buy the Gen 2 I will be “on the clock” for losing future functionality

I wouldn’t be worried. Effectively, there’s no difference between gen 1 and gen 2, and neither are on the “legacy” list. Due, I suspect, to the nature of the fact that the SUB is essentially a slave to another device. For all practical purposes, it appears to take on the features of the speaker it’s bonded to. 

It’s possible that in the future, there might be some software change that impacts this, but right now, it’s not worth the money at least to me to upgrade mine.

I currently have a beam that I will be pairing the sub with. I am not looking to buy rear channels right now as this set up is purely for tv and music from the Beam. I do not intend to buy rear channels at this time. Any thoughts as to whether adding the sub only will make a significant change in tv and music sound?

Yes, it will. 

I’ve got the beam, sub and 2 ones as rear and its absolutely great!

Now i’m trying to sell my beam so i can buy the Arc instead. Was considering selling the sub for gen 3 sub but now i’ll pass that.

Does Sonos still offer a trade in program?

There is a trade-up program, for ‘legacy’ devices. The Sub -- regardless of generation -- is not ‘legacy’.

Yes... A huge difference. I have the Beam and Sub plus two One's (Sonos really needs to change how they name models) and can contest there so much a difference that sometimes I have to put my system in night mode. I happen to like my neighbors.

Your experience will differ depending on your media.  If watch a lot of news and talk shows the sub will not shine here of course but sci-fi and action, it's amazing.  I've had mine for just a little over a year and I was skeptical if this little thing (compared to traditional subwoofers) could put out. I was pleasantly surprised.

As for music, the same holds true.. if you listen to music with bass, you'll be happy. The more bass your music has the more the Sub shows off. I had to actually turn the "Loud" option off in the EQ settings to keep the sub lower at low volumes.

As others, right now I see no need in going for a Gen 3 sub.

The Gen 3 has twice the memory of the Gen 2, which had twice the memory of the Gen 1. However what is less clear is what functional difference this makes in the case of a Sub, which is one of the “dumbest” Sonos devices in terms of what it needs to do.

It could just be that the old hardware is no longer available or more difficult or expensive to source for Sonos due to rapid progression in technology so they decided to “update” the internals to bring it up to par with more recent units.  They did the same thing with the One (Gen 1 vs Gen 2) and there is absolutely no difference.  They both handle voice assistants the same, which is probably the smartest thing they need to do in terms of processing power.  As others have said, the sub is a “dumb” device so I suspect it is nothing more than this and they have to call it Gen 3 because of the change.  

Probably just to handle a higher sampling rate if you have hires audio or Dolby Atmos. Sonically they should be exactly the same. 

Actually I am very curious (and worried) if the Atmos setup from an ARC will require a Gen 3 sub in order to work?

No, Atmos will work on the Arc with both gen 1 or gen 2 SUBs. Essentially, if a device can run the S2 software, it will work with the Arc. Which means only devices on the ‘legacy’ list will not. The SUB, of any generation, is not on the legacy list. 

Can I confirm if Sub Gen 3 have 2GB RAM ? What is the CPU spec ? Thanks !

The specs: https://www.sonos.com/shop/sub.html

Sub Gen 3 has 128M of RAM. The Sub is the simplest of any Sonos speaker so needs less horsepower than anything else.

The specs are wrong then?

Memory
256 MB SDRAM
256 MB NAND Flash

I have yet to buy a sub, my first Sonos prouct (ARC) is on preorder.

But...looking at where I might place it, I have seen no discussion about the huge size difference between gen2 and gen3!?  

From Best Buy website:
Gen 2: 15.3 x 15.8 x 6.2 in.

Gen 3:  18.9 X 19.9 X 8.85 !

EDIT: I talked with Sonos and Best Buy is incorrect! They are the same size.  It almost caused me not to buy it. 

Best Buy is probably listing the size of the box it comes in. 

Those dorks!  They have the correct size for the gen2, so I assumed they knew what they were doing :-)

Anway, since it doesn’t have any vibration, has anyone on here laid the sub on it's side in an open cabinet (with air space above it)? Not optimal, but my options may be limited depending on what #1 thinks of how it looks haha.

I have mine under the couch, lying down, and under the bed, lying down. Can’t see either of them, but they do work rather well. ;) 

 Awesome.  I have an entertainment center with an open center bay, 18”X18”X10”.  So I figured that would be enough air space around it to let the sound come out. Question is...will it rattle the doors. Worst case, I have to move it to the left side of the entertainment center.

Depends on how you set it in the settings. It can, if you ask it to. I’ve got mine set to rattle my bum, occasionally, if there’s an explosion, etc, but not so much that it is overbearing. Which I can (and have) do, on occasion. It’s pretty impressive. 

Adding a few of these can reduce door vibrations.

https://smile.amazon.com/gp/product/B006WP617I/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1