I had an older Laney box with 4x 100W HH speakers, something like that could easily cope with your stereo setup. It weighed a ton though, which is why I stopped using it - only good if you look like Conan or have two roadies available
But as you said, that transistor will be nowhere near 400W in reality, half of that will be heat and noise.
Frankly I don't understand why you want to go with that. Don't you use a PA? Monitors? If yes, then your Marshall power amp will be more than enough for smaller venues. I guess it's probably louder than that "400W" transistor amp anyway.
Carvin have some really nice and affordable boxes, take a look there as well: http://carvinaudio.com/collections/vx-s ... r-cabinets
EDIT: just noticed that they killed the Carvin guitar brand and the old headstock shapes... bad for business IMO. Actually stupid as stupid gets.