I finally finished my comparisons between the io-4 and io-6. I've settled on the io-6.
There were a few things that ultimately lead me to preferring the io-6
1) I like that when using the io-6 with ANC On, I get a better bass response when wearing my glasses as opposed to with ANC off. With the io-4 and io-6 with ANC turned off, I loose some bass response with my glasses on. And I have very thin frames. I end up having to hook my glasses over the ear cups which is fine while at home, but not as practical when out
2) I personally prefer the slight, and I mean slight, boost in the highs with the io-6. Sounds a tad more exciting to my ears. This is with ANC off.
3) I like that with ANC On, the io-6 has a slight bass boost. I find that when in noisier environments, a bass boost is a good thing. Even more so when there's no ANC tech involved. For me, the io-4 struggle a little in noisy environments. Not that the io-6 is as good as the Bose N700, Sony 1000XM3, or B&W PX7, but it's still noticeably better than the io-4. The io-6 gives you some ANC and a bass boost which really help. There's also a slight volume difference, which leads me to #4
4) On my Apple devices, it feels like the io-6 is about 1 click louder than the io-4. To my ears, it sounds like all frequencies have a slight boost overall. All though I believe Brintkiks denied this a few pages back, it sounds to me like the io-6's amp is slightly more powerful. Could just be my ears.
5) Finally, and this one surprised me the most, I noticed some distortion on the io-4 with the power switched on when gaming on my PS4. I didn't notice this on the io-6. At first I equated the issue to burn in, but after a week or so, there was no difference. I connect to a Sound BlasterX G6. During explosions it sounded like the io-4 was really struggling to display the impact. This doesn't happen for me with the io-6. I'm wondering if my io-4 was defective.
All in all, my preference was to prefer the io-4. I already have the B&W PX7 and AirPods Pro for on the go use. I don't necessarily need ANC on the Dalis, and saving the $100 would've been nice. It ultimately came down to my slight preference in the io-6's sound quality, and the slight distortion and volume issue of the io-4. I didn't really feel like hassling with exchanging the io-4 in hopes of finding a unit with less distortion, I just took it as a sign to stick with my io-6 as I loved them from the jump. Also, having the option to use ANC to block noise around the house, and a nice carrying case are definitely bonuses. Just from having ANC, there are times where I will take my io-6 out an about. No matter which you choose though, they are both amazing headphones and Dali should definitely be commended for this amazing first effort.