Thanks Bernard - I know about that one. Luckily not from personal experience, but I know a man who did! I considered using plastic bolts for safety, but they're just not meaty enough for the job. I'll probably end up using a self-threaded fibreglass stand-off instead of a metal bolt, to make sure there's no chance of someone opening it up and shorting it out accidentally.
Richard, that makes a lot of sense. I *could* do a proper hi-Z option in the megohms, but it's all more hassle. OK, may be better to do one thing well. DI boxes aren't incredibly expensive anyway. And as you say, it gives me something to do for my next project. ;-)
Foolestroupe, that's exactly what I'm doing in my prototype. If you're bolting it down, the bolt head sticks out a ways unless you've got a dimple pressed in the base, and that'd get in the way of other rack-mount gear. So I'm using self-adhesive pads and cable-ties to keep the toroids in place. I don't think it's a great solution for commercial stuff though, especially if it needs to stand up to heavy treatment. The sticky pads are eventually going to give up the fight, and then you've got a heavy transformer (and attached mains cables) whanging around loose in the box! Not good news at all.