I agree with the others that you already have a nice system, and that your room presents a distinct challenge. Since there seems to be no way to alter your listening room, I'll only address the issue of components. If you have decided that it's time to upgrade, then here are my comments and suggestions:
1. Speakers are the only transducer in your system (unless you have a turntable/tonearm/cartridge combo, in which case the cartridge is also a transducer). Transducers convert electrical energy into mechanical energy, or vice versa, and they have the greatest variability in performance of any component in an audio system. Hence, new speakers will produce the greatest audible change in your system. Whether or not you WANT to change speakers is obviously your choice, but this will produce the greatest change in your system.
2. The second place to think about an upgrade is your integrated amp. The Plinius is a good unit, but there are better integrated amps available, and a separate preamp/power amp combo has even greater potential for improvement. For example, if you are willing to consider separates, then you might consider a good tube preamp with a high-quality solid state amp. This would give you the benefits of tube (more important in the preamp where the signal voltages are low), with the advantages of solid state power amplification (better bass control, slam, etc.).
3. Your CD player is a very good model, so you might want to consider adding an analog front end (turntable, arm, cartridge, and phone preamp) to round out your system.
4. Last suggestion: if you have not already played with different models of interconnects and speaker cables, you might do so. This involves the final stage of "tuning" your system, and you may find that different IC's/speaker cables make a useful improvement. Sometimes dealers will let you audition cables at home, but if you don't have a local dealer, you can often but used cables to try that can then be re-sold at little loss if you don't like them.