Below a certain voltage, tube circuits just stop functioning altogether
That's the point, to bring the power low enough to introduce the pleasing intricacies. With variac parameter at ~65%, reducing the power tube grid bias parameter very far to the left brings everything down to crackles, at a certain point they just shut off. Variac parameter range lower than 50% you could leave grid bias parameter alone and still bring the whole amp down into that edge-of-bias break up range, preamp along with it.
Variac range down to 0% would also let you to starve the preamp sections down into that extra breakup range when using them without the power amp simulation, 50% variac doesn't seem to have a very drastic effect on preamp feel when using them with the power amp turned off. Maybe if you were able to starve the IIC+ preamp down to 10% variac nobody would be asking for a studio pre anymore, or if you were able to starve a high gain marshall preamp down to 15% variac, maybe it would scratch the ADA MP-1 itch. Rack preamp breakup always feels so soft compared to heads/combos