+1 on that. If you live in a semi-detached or terraced house, or a flat, 5W is too much. I have the same problem.
I'd just keep to the Marshall and use an attenuator to reduce the power output fed to the speaker cab. I've got a THD HotPlate, Weber MASS and a Marshall SE-100. All work OK, but I prefer the MASS.
There are other really smart solutions, too (take a look at the Guytron GT-100 amp for inspiration. Or even build an amp with Power Scaling.
If you're feeling really adventurous, you can make a dummy load for your Marshall, derive a line level feed from that and then run that signal through a cab simulator (to get the tone of a guitar cabinet), and run the output of that into an Aux input on your hi-fi. That will sound great at all room volume levels.