"Boutique Marshall" is kind of like saying "professional do-it-yourself". A Marshall is a Marshall, and trying to get something else to sound like one would entail tearing out most of the guts and replacing nearly everything except (possibly) tube sockets, and if you're very lucky, transformer(s).
Also, a huge concern with a hi-fi amp is that they usually do not have iso transformers, which keeps hv from traveling up your cable and into your brain.
So, once you gut the thing, and modify it into a "Marshall", you probably will have spent way more than buying an actual Marshall.
You would have a much easier time converting it into a Champ type circuit, and even then, you'll have to be a bit lucky to be able to use what's inside.
Finally, by the tone of your question, I'm betting you've never worked on tube amps. Research heavily beore you go poking around, or it's curtains for you.