i don't know enough about the new one in regards to build and circuitry to draw an accurate comparison. My father owned a 401 years ago, and I loved playing that amp, so after some searches I found one pop up for $300 and I grabbed it. I'd imagine that both amps are somewhat similar, the 40C might have another feature or two (power break for starters), but I only run the clean channel so I just needed the headroom that I knew was great on the 401. The 401 is UK made, and if I'm honest that did kind of make a difference for me, but that's illogical.
I believe that on release, the prices of the 401 were in the $700-800 range, and so the cheap labor has the 40C at the lower $500 mark. This is why you'll see a used 401 within shooting range of a new 40C. Prices for the 401 seem pretty stable and I don't expect them to change, while a 40C from the showroom floor will naturally take a hit as soon as you swipe your card.
I think you'd love either the 401 or the 40C, they're both really awesome amps. A 401 would obviously have to be considered used, but that's totally fine.
some early 401's had a minor overheating issue that was resolved after 2003 or so, and the fix was pretty easy should the amp have any problems. Mine is pre-2003, but i haven't noticed any issues, neither did the previous owner. To help keep it that way, I've basically removed the entire back to get some more air circulating back there. It also lets me see the tupes, which is kind of pr0nographic to me.