I have been using a pair of Quicksilver Mini Mono's for the past 5 years and I really like the sound of these amplifiers. Mike Sanders designed these amps for the el34 tube so he recommends using the el34's, kt77's, and 6l6's. I am still curious how the bigger tubes such as the 6550's, kt88's, and the kt90's would sound in these amps or any other amps in comparison when driving floor standing speakers even though it would not increase the power output. I am currently using Alon 1 speakers and seem to have enough power for reasonable listening levels using the smaller tubes. If anyone has done any comparisons recently your opinions would be appreciated.