Any time power tubes are changed in an amp(providing it is not auto-biasing, as mentioned): The amp/tubes must be re-biased, then re-checked in a couple days. Give the amp(s) at least 30 minutes of warm up before biasing(to allow the voltages to stabilize). If you are concerned about reliability of a chosen tube: Contact Kevin at Upscale. He'll give you the straight scoop, and his testing/screening/burn-in procedures are second to none. (http://www.upscaleaudio.com/view_category.asp?cat=85) KT-88s (or any other tube) may, or may not, be more likely to fail based on where and by whom they were manufactured, how they were handled in shipment, how they are biased, and if the gear they are used in is especially hard on tubes. Lots of variables. You may be interested in these if your taste is for accuracy rather than tubiness: (http://www.euroaudioteam.com/pdfs/eat_tubes_september_2004.pdf)
