I have heard that my Art Tube Pac (intended use for tube warmth then sent to a Radial JDI for Xfrmer ISO for house mix) would sound better if I replaced the stock tubes. The stock two tubes are said to be cheap 12AX7's. I have read that for bass medium gain 12AU7's are better for more dynamics and better bass resonce. Is this true and if so, can someone recommend a brand that they have actually used? It sounds OK to me now, however; if I can get a better sound I will do it. Price range $50 max. for both tubes.