I'm using a mix of the original 3000 series with a 7070si Sub which has been absolutely stunning for me, I cannot praise these speakers enough mechanically. The 3000 series Sub (including the newer "i" version) are pretty bad in comparison to the rest of the speakers, I hated my 3000 sub, the 7070si is on a different level in comparison.
I'm told this "i" series is more cosmetic and minor changes in comparison to the original 3000's, I've not done a full in depth comparison, but a quick stat check shows their exactly the same power and build wise.
One minor gripe with Q Acoustics, is they use stickers as their branding rather than proper logos, whilst my 3010's, 3020's and 7070si have stayed on. Due to where the magnet is on the center speaker (right on the sticker) over time the sticker comes loose. I have read this issue online a few times from owners of the 3090c and 3090ci. Considering the branding is so important to some, to have a flimsy Q sticker on your products is a bit of a bad choice, cosmetically the speakers are amazing, but the center speaker needs to have it's branding somewhere else away from the magnet, or Q Acoustics need to stop cheaping out and put a proper logo on their speakers. I was gutted to see my "Q" logo was coming off my center. Managed to reposition it now, but it's unacceptable.
I messaged them on Facebook for a new replacement sticker around 3 weeks ago, as someone on another forum said they got sent one, but QA hasn't even answered me.