I was unfortunate enough to have a close, opposite direction same level (FL330) airmiss about seven years ago. We had TCAS on the a/c but it was u/s at the time and not mandatory in Europe. Luckily we were VMC and I saw the guy coming.
The only point I would like to make is that it is very difficult to visually judge the relative level of other traffic, especially now with RVSM and 1000'separation. It is even harder at night and sometimes cloud layers can add confusing optical inputs.
My condolences to all affected.