Tung-Sol KT170
Due to the success of the KT120 and KT150 tubes and the popularity of these tubes with High-End Audio manufacturers, the Tung-Sol KT170 tube has been developed to provide a beam tetrode with even more power capability. The Tung-Sol KT170 has a power dissipation of 85 watts allowing amplifiers using a pair of these tubes to reach power output levels in excess of 300 watts.
The Tung-Sol KT170 has an original shape glass bulb that was designed specifically to maintain the highest vacuum and dissipate heat when the tube is operated at high power levels. The Tung-Sol KT170 also has an equipotential cathode for increased reliability and to effectively amplify audio frequencies in the output stages of High-End Audio equipment.
The Tung-Sol KT170 is the largest and most powerful tube in the 6550, KT88, KT90, KT120, and KT150 series of audio tubes.
PrimaLuna's Blog 
Which PL amplifiers would take this new tube?
So will this KT 170 work in my Prologue Premium Integrated Amp?
Are these these available thru Upscale Audio?
Would the KT-170 work with your HO dialogue premium integrated?
Interesting!
Can these be used in a Prologue 5?
Its a little strange that nearly a year after the kt-170 Primaluna/Tung-Sol joint venture, neither company can confirm if this tube is even compatible with any PL equipment. Obviously the Evo 400 lacks the physical space, but the 300 appears to have the room. Makes me think this was developed for future products and not the current lineup.
We once got 2 samples, none or hardly any technical info and that together with being more busy than ever, resulted in little or no attention from our side for this, Sorry.