1. Do I need two attenuators? My cabinet has two speakers (8 Ohm each), and SC20H has two separate outputs for each speaker, so does it mean I have to buy two attenuators, one for each speaker? I didn't manage to find SC20H schematics, so I'm not sure if those two amp outputs are connected in parallel internally, and I can just use one attenuator.

A single attenuator will be fine as long as the total Impedance matches to both the Impedance of your amp and cabinet.

1. Do I need attenuator(s) at all? I want to run the amp with overdriven tubes, thus I have to set max level, right? Consultant at my local store told me it's enough to reduce "Master" volume, and EL34 will still be overdriven, if I set "Pre-amp" volume to max. But folks at Marshall forum say that it's still better to use attenuator, even in 5W mode. Again, I don't have SC20H schematic, so it's hard to tell who is right.

Due to it’s design, running the SC20 in low power mode will still generate distortion in the output stage so that the amp still sounds ‘big’ at lower output levels.

1. What if my cabinet only have one 4 Ohm input (2 x 8 Ohm speakers inside)? Is it enough to connect it to the amp with only one cable? Or I have to rework my cabinet to have two different inputs (separate input for each speaker)?

If your cabinet is rated at 4 Ohms, simply use a single cable and plug into one of the 4 Ohm sockets on the right hand side.

Kind Regards

Marshall Support