Marakanetz, I just wated to clarify your statement. If your are talking about a TUBE POWER AMP, then the most critical (as well as affordable) sound comes from the low level driver/input tubes. A VAST sonic improvement can be gained by replacing the stock Soviet, Chinese, or Yugo tubes with GENUINE NOS tubes (Euro or American...'70's or earlier only!!) Sure, replacing the power output tubes would be nice, if you could find the quantity (4 and up to 16 or more that may mono-blocks require) and the price! Also, low level tubes will have a lifespan of 2 or 3 time greater than that of a power output tube.