BoredSysAdmin is right. First of all, your amplifier is not underpowered. It is rated for 100 watts per channel, 2 channels driven. Your speakers are rated for no more than 150 watts. That is a difference of 1.8dB, which is a small difference. And that is assuming that the Sony cannot exceed its rating, which it likely can into 2 channels.
Second, there is nothing wrong with the DACs in the Sony. So there is no point in buying a separate DAC. You may as well flush your money down your toilet; that will have about as much affect on the sound of your system.
Third, a good subwoofer would make a very big difference in sound quality, as it will give you bass that your current speakers cannot give you. However, since you live in an apartment, there is a good chance that a subwoofer will bother the neighbors, and so you may not be able to use it enough to make it worth getting.
Basically, if you are unhappy with the sound you have now, you should first consider changing the position of the speakers or listening position, or consider changing the room's acoustics. If that does not help, then you should consider replacing the speakers to something more to your liking, or adding a subwoofer if you want deeper bass.