I haven't tried those exact ones but I do currently use the Groove Tube version of them, the Substi-Tubes.
They really are a great and fun idea. I can't really comment on the THD ones but the Groove Tube ones are all metal and there is not ground wire you have to screw down, just pop them in and you're done.
To be quite honest though I was expecting the volume drop to be more significant; I'm using mine in a Sovtek Midget 50H, 50 watts not about 25 watts. There is a difference in sound I've noticed though, maybe in part because the 6L6's that were replaced with the EL84's were ridiculously old.
I think they are worth it though, even though the volume decress wasn
