I was not saying that the absence of Livia and Pussy were the sole reason the show went downhill, it could have been mere coincidence. Other than Furio, they failed to introduce likable (or hateable in a good way) new characters, with maybe the exception of the expansion of Phil's character, to pick up the slack. Plus, I think most shows only have a few good seasons in them before they start to go stale, and very few have good ones late in their run.
I never noticed the accent change in Tony for season 6. Now I'm going to have to go on On Demand and do a compare and contrast.
I think part of the reason the show went downhill in my eyes is because Chase went out of his way to make the characters less likable. Who didn't enjoy when Tony took care of the informant on Meadow's college visit, or when Paulie and Christopher took care of Palmice while he was jogging. When Tony whipped the city councilor because he was banging his ex-goomah after it was he who called it off in the first place, or when Paulie snuffed out the old lady, or when Paulie and Christopher killed the waiter, they were no longer cool. Your avatar, despite being a serial killer, remained cool for almost the entirety of the series. Deb, on the other hand, went from being very likable to detestable (almost as much as Rita) the last two seasons.