I dont own a model from the 9900Z...3900Z Line so I cant really give you a guaranteed answer to your question on the sound quality. However, I can tell you that I do know someone who is another Sansui enthusiast and he's very knowledgeable- he's just too busy to be able to spend any time on this Site - he has a quite a collection of Sansui amps, receivers, speakers etc...... and he happens to have quite alot of models from the Z line. We have corresponded some time ago and discussed the Z models- he feels they are pretty good units. Since I do value his opinion, then if he feels they are good, and he does have the electronics knowledge to back it up, then I'm inclined to go along with him and thus I would say if you are interested in getting a 3900Z, then I have no objections to tell you that you shouldnt.
I presume that since you are in Asia, you dont have as much choice to select what you like from whats available on Ebay and instead is looking to acquire a Sansui that happens to be in your local area. If thats the case, getting a 3900Z should be fine but since its the smallest model in the line, then I'd try if I were you to buy it for only a modest price. And yes, with decent speakers that are efficient, I think the 3900Z should be ok in offering enough power capability.
B/F.