I owned PL Prologue Two, upgraded with NOS GEC KT88, no probs with mids or anything else. For the money, and not only, PL gear are stellar, imo. And KT88 are very musical tubes, powerfull, but still...And EL34 are great option as well, but they are not the same, less punchy, more relax, so, its all depends on your preferences. You know, the best part is, you can use KT88 and EL34 on it, even at the same time, amazing futures PL offers with auto bias. I tried KT88 and 6550 together, no issues. I dont know any other amp that can do such thing.
Enjoy the music!
Enjoy the music!