I have been in this market for quite some time now. I too have the Z-2300, and with being in an apartment, I have to keep the sub turned way down, and as you probably know the Z-2300 have muddy mids and highs.
From what I have read, hopefully this helps, the quality increase lies in a good DAC (digital/analog converter).
As was mentioned previously, a used audio receiver does this quite well, and you likely have a digital audio out connection (optical/coaxial) on your computer. Doing this opens up a world of bookshelf options for you.
You can buy a standalone DAC, they make USB ones, but a good one is expensive. If you have the space to put a receiver in your office, then that would probably be the best value option.