I have been playing with the tube preamp implementation in my SX-780 some interesting results: M5214 op amp with a gain of 33 is lower noise then an op275 gain of 6. the tube circuit consists of a 6922 with 10k plate resistor and 560 ohm cathode resistor. No bypass capacitor. this config is able to put 20v p-p into the tone stack with an apparent gain of around 8 while using a B+ of only 45 volts 6922 also appears to generate less noise. is there an easy way to calculate distortion or does it need to be measured? Setup with tube sounds fantastic.