It's not a matter of louder, it's a matter of better. You lose a lot of dynamic range as the Sony ES you have now is current-limiting. If you get any deep lows (which is not 100% dependent on volume), the receiver will current-limit and you will have a loss of quality.
It's not going to hurt you, but you are not running 100% of what the speakers are capable of, and can possibly damage your speakers given the right circumstances. The Sony ES receivers were only designed to tolerate 4 ohm loads, not run effectively with them. I had a 555ES and it worked with 4 ohm loads (Magnepans), however switching to a dedicated amplifier opened them up to their full potential. Remember, it's not just volume, it's dynamic headroom, and current. Also - your receiver isn't really discrete, since it still only has one power supply shared among all the amplifiers in it. No receiver really is.