Thanks for the tip @flocked As mentioned above, I have unfortunately deep power cycled many many times (maybe 20 of the 200ish power cycles have been after 10 min - 8 hr breaks?). I’m literally on a family holiday in the middle of nowhere (with everyone else watching cricket on TV) and the only thing I have with me is the Digitakt - A deliberate plan for me to learn how to use it! I’ve been plugging it in and turning it on at random intervals all the time in case it mysteriously comes good. I’ve given up hope it’s going to work for NYE and am rebuilding the set in Octamed (for the Amiga) which will get me though the NYE show. I’ll return the Elektron under warranty when I get home on 2nd. There’s only so many times you can flash the OS / reboot only watch it freeze in exactly the same manner every time before you realize it’s cactus!
No multimeter to check the PSU under load where you’re at I take it? That would be my first test after reflashing / downgrading etc. Make sure the external PSU brick is not defective.
I’m sorry to hear that the DT isn’t working, but using OctaMED on an Amiga is damn cool! Please tell us more about your setup and set you’re doing with it! 
As an update to my particular no-boot - I took the Digitakt back to the store and showed them the fault, showed them this thread, and they replaced it with a boxed new one… no questions asked. Only a comment I got was that they’d never seen any Elektron unit behave in this way. Now to actually make some tunes with it… (!)
Oh wow, that really, really rocks! If you don’t mind me asking; what display are you using there?
FWIW, OctaMED works pretty well synced to MIDI clock, and it also is a pretty decent clock source. Bringing the DT into that setup should be reasonably straightforward.
Also, you might be interested in https://forum.mutable-instruments.net/t/have-yourself-a-20-years-old-8-bit-amiga-sample-pack-now-with-16-bit-44-1-khz-wavs/4637
Not massive at all, just thought you should check voltage, and checking voltage without some sort of load can be misleading. Sounds like you need customer support before heading into warranty voiding territory. 