Articles are always tricky and confusing. I had read somewhere that even scholars make mistakes putting articles at proper place! So, don't worry, we are not the only ones!
I also read Swan's. It's a good book and I remember the portion of the music of 1960s.
Anyway, I'm a non-native speaker and find it difficult to identify where to put articles and where to not. However, in another reputed book of grammar and punctuation, I read that fortunately a sentence without articles is fine and generally understood but if we learn it perfectly, it's good for us. Here, I'm trying to give the answer that I think and follow.
The definite articles have myriad of rules and with practice only we can identify where to put them and where to omit.
Now, your case.
The readers of a magazine
The moment you put the in front of readers, it'll specifically talk about the readers, identified readers of some magazine (because or else it could have been *the readers of the magazine).
On the other hand,
Readers of a magazine
Both are unknown. Some readers of some magazine.
Let me build an example. The magazine's name is ELL and ELU. Some readers (ELL) are wearing 'blue' tee, some (ELU) are 'red' and others (No choice) 'green'.
In the big hall, the readers of the magazine (ELL) were at the left side of the stage. This surely means that the readers in blue-tee are the readers of ELL and were on the left side of the stage. Same will go with the readers of the magazine (ELU)...on the right...
In the big hall, the readers of a magazine were having fun talking with each other. This means the readers in blue, red and green tee were talking to each other and were having fun. Because here the means those who are in the hall but then a magazine is any magazine. On the other hand The readers of the magazineS were enjoying with each other - all the readers of both the magazines were enjoying with each other.
In another case,
The readers were not interested in any of those magazines - certainly, which readers? I'm talking about the readers in green tee. The identified them as they are the special group of people in green-tee, not interested in ELL or ELU.
Readers in the city were too enthusiastic to attend the function at the big hall.
We are talking about readers in general. Mind it, they are neither those blue, red or green ones or anyone in the hall. They were the readers in general who could not attend the function.
The same can go with your concert example.
Note: There are many possibilities to come up with the example of the hall. We shall explore as we discuss on this.
This is strictly my opinions and let others come with their stance. Again, what I said should not be considered as a concrete rule as articles are always a big trap (at least to me!).
Note: Follow Damkerng's comment. That's the general use. When something is previously introduced, it'll take the definite article. I found a herb in a garden, the herb was green.