In that case, I would say Stadium for your next purchase. It will do so much more for you as far as both amp tones and effects. You can use it on its own or in front of the Twin to add effects.
My next purchase suggestion would the. be something like a Ceriatone Plexi if you want medium to Van Halen level gain. I am partial to the all access, which gives you a variety of Marshall options, but most of their Marshall style amps come with or can be ordered with an effects loop. That would give more flexibility with the Stadium and also let you use it as just a clean(ish) power amp for the Stadium.
If you want more gain, the hotter 2 hole Marshall variants including the modded ones come into play, or a modern higher gain amp. Most will also have effects loops to pair well with the Stadium.
A Twin, a Stadium, and a Marshall style head plus a couple cabinets and maybe a Fender FR-10 would be an amp stable that can do everything you need and almost everything you could want.