Re: Marshall 4104 2x12 Combo
You cannot, to my knowledge, use an A/B box to switch between the inputs.
If I remember my combo model numbers right, this is the 100 watt combo version of the 2203. You will most definately be able to get some nice high gain tones through the high input...without a pedal. A pedal will take you further into high gain territory.
There are a lot of options for getting the sound you want out of a single channel Marshall. Setting the amp to the max clean headroom and then using a pedal like the Fulltone Fulldrive II which has an overdrive setting and then a solo boost on top of that....it's like having a 3 channel Marshall. Or, you can have it modded by someone like Splawn amps, or Jeff Seal, and they can actually turn it into a 2-3 channel Marshall.
The key is how much can you pick it up for. I have seen them going for crazy $$ on eBay. I paid $550 for mine about 2 years ago. I have the 4212, which is the combo version of the 2205. I absolutely love it, and had it modded by Jeff Seal.