In terms of sound signature, I say they sound fairly similar. The H7 have more perceivable bass, but I just think that it feels that way because the bass is not as tight as the H6. The H8 manage an in-between, where they have powerful bass, but still manage to get some detail. However, neither the H7 or H8 bass sound as good as either H6 model surprisingly (to me of course).
However, the H7 still sound pretty grainy to me. Personally, when it comes to sound quality, wireless is not an option. When I connected the cable to the H7s and my iPhone 6, it didn't improve the sound as much as I thought it would. They were still really quiet un-amped, and they still had a bit of graininess wireless headphones tend to have, and lacked that detail I expected would be gained once I had a wired connection.
Overall, they are some of the better wireless headphones i've heard by far, but wired, they don't have the clarity, smoothness, tight bass punch, and detail of the H6. I would say they have similar signatures, but the H6 sound better without a doubt.
The bass quantity on the H6 Gen. 2 is close to the H7, but much more tight and detailed. The bass on the H6 Gen. 1 is my favorite, but is completely lost when your environment is loud.
H6 Gen 2 sound in loud environment ≈ H6 Gen 1 sound in quiet environment
It's like having two sets of amazing headphones lol.