The need for an attenuator depends on whether you want it's own sound or just a clean sound. If you want the amp's own sound, you do need an attenuator...the 5W is VERY loud 5W. But a simple L-pad will do well. I had a 20W resistor for that with a potentimeter for adjustment, really easy to build.
I'd recommend some mods, they really make the amp come alive...mostly the 'input mod' if you want some nice crunch out of it...super easy to make too, and even so it's made reversible.