I am very fond of Marantz myself. Excellent choice there. As far as subs go, I am not exactly sure what is available there where you are. Here, there are excellent subs available from the likes of Hsu, SVS, PSA, Rthymik, and Emotiva just to name a few. Buying internet direct usually yields much more for your money.
If the sub you have now is working, then why change it? It sounds like you like it, no? However, going duals would allow you to smooth things out some (less standing waves) and add more headroom to the equation so to speak. Perhaps, adding a second Yamaha 8" sub identical to the one you have may yield good results. On the other hand, moving up from a Yamaha sub to any from Hsu, SVS, Rythmik, and etc. mentioned above will certainly be a significant improvement. Especially, if you went with duals.
How big is your room? What is the layout? Is your room open to other areas of your apartment? More importantly, how far is your listening position from your front-stage? Where do you have your Yamaha sub placed now? Lastly, what kind of budget are we talking about here? Need a bit more info first, before recommending something more specifically. In short, I am very fond of sealed subs myself.
Cheers,
Phil