The problem with using the jam knot, Ard, is that there has to be pre-determined 'match' between the leader you are using (and thus the loop to the hook, and the jam knot size) and the id (internal diameter) of the rear of the tube you are jamming the knot into. Secondly, inside the tube, there should be a step at the rear beyond which the jam knot cannot be pulled, otherwise, though the period of casting with your tube fly, the jam knot may creep further & further into the tube rear.
There are several other 'competing' methods of securing the hook at the rear of the tube fly, and all may be appropriate at different times, with different diameters and types of tubes, the size of hook used, and the way they are fished.
Tubes can be fished with a free-swinging hook, that is the hook itself is not secured within the rear of the tube at all; there are even plastic/silicon 'cone' like devices to add to the hook eye/shank to facilitate this true free-swinging.
The hook at the rear can be attached directly at the rear of the tube with a standard hook-eye knot eg Trilene, improved clinch etc, or on a loop on the leader. It is with the loop that the 'jam' knot appears to work best.
Between the loop knot for your hook, and the rear of the tube, you can slide onto the leader (before you tie your loop) a bead (or 2 or 3!) which has a hole in the bead smaller than your loop knot, and the bead od is larger than the id of the rear of the tube, and the bead prevents the loop knot being pulled into the rear of the tube, leaving the loop and your hook free swinging at a determined distance behind the rear of the tube - works well for short tubes with long wings!
As you mentioned, you can add junction tubing (usually softer tubing that that of the tube itself - perhaps silicone, rubber, neoprene or PVC) onto the rear of the tube, and unless you whip this onto the tube with tying thread at the time of fly construction, then there has to be a tight stretched fit between the junction tubing and the rear of the fly tube (unless you want the components to separate). As the junction tubing is usually softer & more flexible (deformable before damage) than the tube for the fly, it is also more forgiving to be able to accept and secure a wider range of hook eyes at the rear of the tube fly. Such junction tubing also is available in a myriad of colours including clear & fluo. It is usually replaceable if damaged.
The 'modern' "easy" method is to have a pre-fabricated stepped tube (eg Pro Tube) with a wider id at the rear to accept hook eyes directly, and thus do away with any and all of the above; however, the major drawback is that there is really only a narrow window of hook eye sizes which will fit snugly & securely into the rear of these stepped tubes; if you use the same small range of hooks for the rear assembly of your flies, then this may not be an issue.
Mike