Seems to be a problem with speakers 9.1 rather than amp since the amp played well with other speakers. Need of watts depends on how loud you want to play. Usually 20w is more than enough for regular listening.
Going by your description, tweeter in 9.1 seems to be problem. Take an audition again(should be possible since the seller your friend) and keep your ears close to tweeter to see whether it is playing or no. Check both speakers. Even 1 non working tweet makes the mix poor. Just take care that volume is not high when you try this. otherwise ears may get damaged.
Dont worry about 6 ohm impedance. Even 4 ohm is fine as long as you are not pushing amp to its limits for long time.