I have Golden Lion KT88 as output tubes and they are great, they are factory original from Jadis. Never heard the amp with other tubes that KT88. You should be able to install them as well if you want, but manual bias is needed! I recently swapped the small 12ax7 tubes for 5751 tubes that are similar to 12ax7 (no need for manual bias) but the difference is not huge, I think the output tubes are more important. I love this integrated tube amp, lovely to look at and the sound from it is beautiful as well!