Craig,
Kestral 2s love tubes and either choice will probably be just fine. On my experience with the Kestrals, I'd opt for the KT88-based Prologue 2. The speakers will appreciate that extra bit of power, and the added "bottom" provided by KT88s will be most welcome, especially if you're listening to rock or big symphonic works. Plus I recall that the Prologue 2 will also take EL34 tubes should you ever want it to (but not vice versa, I believe).
Kestral 2s love tubes and either choice will probably be just fine. On my experience with the Kestrals, I'd opt for the KT88-based Prologue 2. The speakers will appreciate that extra bit of power, and the added "bottom" provided by KT88s will be most welcome, especially if you're listening to rock or big symphonic works. Plus I recall that the Prologue 2 will also take EL34 tubes should you ever want it to (but not vice versa, I believe).