The Red Roo 'Rockstar' guitar amplifier's standard configuration is for push-pull Class-AB operation giving about 20 Watts RMS of power. However, with a different output transformer and some reconfiguration, it can also be configured for single-ended Class-A operation. 

So, why would you choose Class-A. Most low-power guitar amplifiers are single-ended. i.e. they have a single output tube or several tubes connected in parallel. They operate in Class-A where all output tubes conduct through the entire signal-voltage cycle.

In a push-pull Class-B amplifier, each tube only works during half of the signal cycle. A Class-AB amplifier is similar but has both tubes turned on slightly all the time. This overlap helps reduce a problem called cross-over distortion, which happens in Class B amplifiers when one tube turns off and the other turns on too abruptly. The more the tubes stay on together, the less distortion there is.

Single-ended Class-A tube amps often create a warm, rich, and natural sound which is richer in even harmonic distortion. They can add a smooth, sweet distortion at lower volumes, which many guitarists love for blues, jazz, or classic rock. However, these amps may not suit all music styles or situations. They usually aren’t loud enough for big stages and might not provide the heavy distortion needed for hard rock or metal. They also may struggle to deliver completely clean tones at higher volumes, which some genres require.

Ultimately, the choice of an amplifier, whether push-pull Class-AB or single-ended Class-A, comes down to the individual guitarist's preferences, playing style, the specific sound they're seeking, and the contexts in which they'll be using the amplifier.

We suggest you start with the standard push-pull configuration and experiment later on. Single-ended operation requires a different type of output transformer, different output tubes (2x 6V6), and some changes to wire links on the PC board and will give about 8 Watts RMS output.