All PrimaLuna amplifiers come equipped with either EL-34 or KT88 power tubes. However, you can substitute in any 6550, KT66, 6L6GC, 7581, KT90 or EL37 into any PrimaLuna amplifier. 

The ProLogue & DiaLogue One, Four, and Six amplifiers can use 6550 and KT88 tube types without any issues. The tubes will be running at a slightly cooler bias, but many customers have been using these higher dissipation tubes with great success. 

Each tube will have a different effect on the sound. Adaptive AutoBias will adjust itself to the new tube type, making experimenting with different power tubes very easy. In fact, the Adaptive AutoBias circuitry is so advanced you could even plug in one 6550, one EL34, one 6L6GC, one KT88, and Adaptive AutoBias would recognize the differences, make the appropriate adjustments, and the amp would function perfectly. Now... it may not sound very good, but it's just nice to know you could.