you obviously had some sort of short, or the fuse wouldn't have blown the first time, and then immediately the second time. my xbox was used and refurbished - and it has the xbit. i've used it for about 3 months, sometimes on for 6 or 8 hours at a time, with no problem.
any time you have a fuse blow like that (car, house, etc.) there's a reason. i doubt it was the mod chip, but maybe a wire, or maybe the mod chip shifted shorting some stuff it shouldn't have.
maybe something jammed a fan causing excessive voltage...
or maybe you had a power surge one time before, damaging stuff. still it sound to me like you had a short.