The right amp is what makes for bass on a Sound Lab. Generally speaking, transistors are not a good match. The reason is the Sound Lab, like most other ESLs, has a 10:1 change in impedance over its range, and is over 30 ohms in the bass, making it hard for transistor amps to make power, and it does need the power.
So if you have a transistor amplifier of 600 watts, it will make about 150 watts in the bass region, which means a 150 watt tube amp will be able to keep up with it no problem.
Set up correctly I've heard these speakers shake the walls and I do wonder if a sub is needed, since they can go to 20Hz all by themselves!