I've been wondering about this myself these last few months. I read a review on the Monarchy M24 DAC & Linestage in PFO and one criticism the reviewer had is its built-in volume control.
"...Is there a weak point to the M24? Yes. It's the built-in volume control, which knocks down the quality of the linestage quite a bit....If you use the Linestage with the built-in control, it'll sound noticeably more closed-in, less dynamic, and less open than it really is, with just a bit of grain at the top. With an external AVC or TVC feeding the linestage, the linestage actually sounds better than the AVC or TVC by itself, and the sound quality leaps to the top of the tube-linestage league."
What is an external AVC or TVC by itself? And who makes such components?
"...Is there a weak point to the M24? Yes. It's the built-in volume control, which knocks down the quality of the linestage quite a bit....If you use the Linestage with the built-in control, it'll sound noticeably more closed-in, less dynamic, and less open than it really is, with just a bit of grain at the top. With an external AVC or TVC feeding the linestage, the linestage actually sounds better than the AVC or TVC by itself, and the sound quality leaps to the top of the tube-linestage league."
What is an external AVC or TVC by itself? And who makes such components?