normally the latest model should be better!!
but 656 also not bad, I set up 3 of them for my friends, and i think for beginner level amp , onkyo 6 series from 646 up is all really high cp, I used to have 646 before lx 89 and rz3100, many of other c hing came to my house before and said 646 really doesn't sound like beginner level amp, But I use JBL speaker and maybe thats y it helps a lot with the sound performance!! but anyway if for 676e and 656, then for sure I will go for new model if the price is not with a big different!!