Re: ohm
The marshall should have a selector switch on the back allowing you to select 4/8/16 ohm impedence levels, get the avatar cabinet rated at 8 ohms (two 16 ohm speakers wired in parallel). Plug the cabinet in, set the amps output impedence at 8 ohms and your ready to rock...... in the future if you get a second 8 ohm cabinet you'll need to select a 4 ohm impedence if you run them in parallel (using the second speaker jack on the amp or plugging one cabinet into the other - avatars come with two speaker jacks on the back for running an extension cabinet). Hope this helps.