Hi Charles,
I have heard TVCs and I do think that they are the best way to implement a passive linestage. I also do wonder if they would be even better if incorporated into an all out active linestage.
My main "problem" with TVCs are practical/ergonomic issues. Most of the available TVCs are not switched by remote control and I really don't care to live without at least remote control of volume. The other common problem is that most have far too few steps of attenuation. I am not a fan of stepped attenuators with 2db or so steps-the "right" volume just seems to always be somewhere between adjacent steps.
Also, unlike most systems, mine actually needs active amplfication from the linestage. My headphone amp is surprisingly low in sensitivity and I need the additional gain when I play my records (phonostage with 60 db of gain, and a cartridge with a .3 mv output).
A friend of mine built a remotely contolled linestage that used a light-dependant resistor attenuator. He had the right set-up for a passive (very short interconnects, suitable input and output impedances). I thought it sounded quite good. But, just out of curiosity, we substituted in a pretty decent tube active linestage (capacitor coupled, 310 tube). We all agreed that, with this particular setup, the active was better--more dynamic and engaging. This was a bit surprising because this system was already pretty dynamic sounding (the speakers utilize Feastrix fieldcoil drivers).