A local dealership that sells only tube electronics (some made in-house) sells an in-line transformer buffer. I did not need it, but, I did get to hear what it does. It makes the sound seem more "open" and expansive (more enveloping soundstage), and whether or not this is artificial, it sounded good to me. But, I didn't think I needed it because my gear is stuffed with iron--the linestage has transformer inputs and transformer outputs, and my amplifier has transformer inputs and, of course, output transformers. All of these transformers are quite old Western Electric parts.
I have heard only the Silver Rock transformer volume control and the MyEmia autoformer volume control, but, neither was actually in my system. I have not played around with different forms of volume controls, but, I have heard various passive linestages with different types of volume control, including light-dependent resistors. In my own system, I heard Placette's passive linestage with a 128 step relay switched attenuator, and their unity gain active linestage with the same 128 step attenuator. I bought the active stage; it sounded more lively than the passive even if it had no gain.