Hi Glynnw, I don't mean to butt in but your subject made me think all day about your question. I built my own passive preamp 15 years ago and used 2 volume controls and eliminated the balance control. Everyone has a different furniture layout where they listen so I did need balance.
I kept asking myself what was the difference between Glynns question and what I had set up. I don't mean to be mean but it reminded me of a certain Seinfeld episode when Jerry asked Elaine if shaking up a Snapple was that difficult as he glared at her lightly jostling the juice.
I think every audio nut must like to fiddle with the volume control.
Glynn, I am using a 8 watt little stereo tube Heathkit amp. The AA-32 integrated amp. It has no balance control but instead has a dual volume control. A little tiny knob inside the regular sized knob. When I turn the big knob, say left channel, the little knob goes right with it, the right channel. Same thing when I turn it down. They can both be turned independently also by holding one knob and turning the other, 1964. It came out before but Heathkit called this a dual concentric clutch type 1 megaohm volume control. Mine works great but when I looked into the Tortuga site my volume control is like something on a Model T used in the last Daytona 500. It is a cheap carbon pot but I think it does what your looking for.
Glynn , I was unaware of the Tortuga stuff. I checked the site and it's beautiful! It must sound incredible.
So sorry for blabbing away to you and Morten but here's your response from the peanut gallery.....sincerely Mark Korda.