If I recall correctly, VAC use solid state rectification whereas CJ uses tube rectification. In any case, this would be consistent with my experience of the sound and power of these units. VAC have more power and precision for a given set of watts; CJ more musical and lovely with rich bass.
The "sheet over the speaker" is the experience of underpowering speakers with a tube amp, IMO.
Thiels need current to sound their best. I powered Thiel 1.5 with CJ 11a and it was fine but the switch to CJ 12's brought about an astonishing improvement, far more than would be justified by the increase in power. You can make Thiels loud with modest power but the tonal quality and bass control won't be there. Thiel 1.5 with CJ 11a might work fine in a small room, but it's a risk for a larger room.