Tube Rolling with Onix SP3

I am fairly new to the tube world. I have an Onix SP3 and am overall happy with this very cool amp. I am using NHT VT1.2 speakers and this little amp seems to have no problems powering up these towers. It seems to me that the mid range is a little weak and "muddy". I was wondering if anyone has experience with changing the tubes? What sound differences were experienced?

Thanks Nofat95