Hi Guglieimo,
Welcome to the forum.
The BC's are all recorded with NAB eq. In fact, I'd never found any pre recorded tapes (commercial) that were recorded with IEC eq until the Tape Project.
I used to have an A77 back in the 1970's and just like you, I preferred using the IEC setting for playback of some tapes even though they were recorded with NAB.
Both of these standards were developed pre 1960 so I don't consider either one to be a modern eq.
There's an excellent article on MRL's web site regarding these and other eq curves. There are many excellent articles on magnetic recording there. Some of which I've totally lost track of how many times I've read.
http://home.comcast.net/~mrltapes/equaliz.htmlStrictly speaking, listening to the BC's with IEC is the wrong playback eq. At this point in time, it may, however, compensate for some of the age and wear related deficiencies of older tapes. And when it comes down to it, if you enjoy the music more that way, it's a good thing.