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
There are some constants that can be seen with tubes in general. Some Manufacturer's tubes are more microphonic in the midrange(consistantly) for instance, and some very transparent. Most British made tubes have a sweet coloration(glaze) in the midrange that some people crave. Some of the least colored are the German tubes(ie: Siemens/Telefunken), and would probably give you the midrange presence you feel is lacking. Try replacing your 12AX7s with ECC83's(or ECC803S), the 12AU7s with ECC82s(or ECC802S) and the 6922s with either E88CCs, 7308s, E188CCs (or CCa's). Stick with NOS tubes and either the Teles or Siemens. The tubes in parentheses are specially selected for low noise and microphonics, highly desirable, expensive and hard to find. BUT- well worth the effort/cost. Be certain to replace the preamp section's tubes with preamp rated(low-noise) tubes. I would also try the Tungsol re-issued 5881s(Russian) or Winged 'C' 6L6GC for power tubes. I've had great success purchasing tubes on eBay(carefully reading feedback and tube test results), and purchasing power tubes from Upscale Audio(torture tested and triple matched).
try searching the discussion section at