The TurboHead for the 8AX is the KT5-HA or KT5-BK.
The collar fits on the 8AX/8NX body and allows you to screw on the KT TurboHead with the N5 Lamp Assembly.
The N5 Lamp Assembly is the same output/runtime as the X80.
If you have any of the other TurboHeads such as the "T", "SRTH" or "12PM" TurboHeads they will also fit and function on the collar with the N5 Lamp.
The N5 in the KT5 on my 8NX is actually a bit of a surprise. Because the N5/X80 filament is pretty small, the N5 produces a small, round tight beam.
More to follow, I have to go offline for a bit....
Al