EL-34, KT-88, 6P3P: What's the Bloody Difference?

Do some research on this in the Audio Asylum tube forum, and/or post this request there. You have a better chance of getting some good responses. IMHO the design of the amp is more important to the sound than the tube's designation. However KT88's, 6550's, and KT 90's, for example are large power tubes and unually produce up to twice as much power, ergo when a manufacturer wants to build a high power amp they choose these tubes. When high power is not an issue they use EL34's etc. The Primaluna is an exception and I think that the use of the KT88 may produce a tighter and more powerful sound even though the power delivery isn't that much different on paper. Why not call Kevin Deal and ask.

You can use a variety of tubes in both the Prologue PL1 and the Prologue PL2. Below is a list that is outlined in the PL2 Owners Manual.
I also believe you can use the KT 120, but I would confirm that with Kevin or Jarred at Upscale Audio.
Should you be interested, here’s a link to the PL1 and PL2 Owners Manual. http://www.primaluna-usa.com/owners-manuals
12AX7: ECC83, ECC803S, E83CC, 7025, CV4004. You can also use a 5751, but please note that the 5751 is a lower gain tube than a 12AX7 and the sound will be altered. • 12AU7: ECC82, ECC802S, E82CC, 5814, 6189, CV4003 • KT-88: 6550, KT-90. You may also use EL-34, 6L6GC, 7581A and KT66 though power will drop slightly.

Just a suggestion, you may look at the Prologue 3 and 5 combo.
I think you can get them both on the used market for around $1,600.00 to
$2,000.00 pending condition, use, age, etc. A very good value and a significant step up in SQ from the PL1 and PL2. (based on personal ownership and use)