8x8 isn’t a bad opening offer at all. That’s the Couture contract who is a very similar caliber of player and probably the closest comparable. With how Duchene has played this year it’s probably going to need to be higher but it’s at least in the ballpark.
The tricky part for the Sens has always been the signing bonuses and upfront money, Couture for example is at 10, 9, and 9 his first three years. Even a guy like Evander Kane is at 9 million his first year and 8 the second despite having a 7 million AAV.
If Duchene gets an AAV of around 9 I think the Sens could do it, but if he also needs a big signing bonus and 11 his first three years that could be tough, especially considering Stone is likely to get even more.