@FJ1000 I think the powervalve is only available with a full system, I may be wrong though.
As to drone, I have a turbo back powervalve sport system with 200 cel cat .and can confirm that there is zero drone at all. It passed mot last month no problems, and even the techs in my work are taken with the quality and sound of the system.
I am so impressed with the system that I would probably only ever buy future systems from them.
Hope this helps