I too like using Pratt sounds,I like to use sounds that are long and heavy,as you say,it is a great sensation to feel the weight inside your cock,I used a lube twice,the second time,I got a UT infection,since then I use my saliva,no problem.as yet I have not managed to cum using sounds.just checked out your video,you certainly got a load of thick cum out of your balls.
I use the Pratt sounding rods, I'm up to a 9.3 mm diameter. I'll be at a 10 soon, I love the feeling of sliding in the sounding rod and letting the weight of it slide down my shaft. Most of the time I don't use any lube, my cock creates a lot of pre-cum and that's the best lube of all. It's always great to cum off with it inside my shaft, it really heightens the experience