I have 16 KT-170's running safely in my pair of PL Dialogue HP's. Its just a mater of raising the rear tubes up about 2 -3 inches. It would be nice if PL made a nice audiophile grade adapter to do this, but $5 socket savers plugged in together does the trick nicely. The side to side dimensions are just fine. Its the front to back dimensions where you have glass on glass touching. I crammed them at first to try them out against the 150's and the sound was everything I liked about the 150's but more so. Better extension on both ends, better midrange. I have a small mid bass bump in my setup at the moment and these helped alleviate it somewhat. The rear KT-170's are raised 3" higher than the front ones and it gives plenty of room for heat dissipation, and they are well away from the front tubes, so no glass on glass. The socket savers are $20 for 4 on amazon so you need 6 sets of 4 for 24 socket savers at an additional cost of $120 for the 8 rear tubes. BTW If anyone is interested in 16 low hour KT-150's (about 300 hrs in a Dialogue HP). I'm selling them at half price and will include 2 spares I used in my headphone amp. I have many dresser drawers full of tubes I am not using and need to clean house.
Brad