Sound improvement

[stauburn]

I am looking for suggestions to improve my sound in the vehicule. I have the stock radio in my 05 SR5. What i want to achieve is a fuller cleaner sound . I need to use my cargo space so I do not want to put a sub box in the back. I haven't heard the JBL sound system but if I assume that it sounds like the BOse system in my cousin's QX4 that is the type of sound I am looking for.

Thanks in advance.