Taking the six in order, the first is the first to strike off the list. Its principal attraction, one imagines, will be to people who have a particularly happy memory of the Franco Zeffirelli film (staring Fanny Ardant and Jeremy Irons) not yet released in the UK.

Little information is given about it: nothing, for instance, to place the excerpts from the soundtrack within the context of its action. The items in which Callas sings include the ‘Brindisi’ from La traviata as performed at La Scala under Giulini in 1955; the others are standard studio recordings better heard in the context of the discs from which they are taken. The three orchestral pieces by Alessio Vlad are sweet-sad nonentities, which may gain a value by association with the film, but for anybody who is as keen on it as all that, it would surely be better to seek out a DVD.

‘Callas Live in London’ should likewise disappear from those lists where good recorded sound is a priority. Conversely, it could find its way to the top of lists kept by romantics, people who feel they might like Callas more if she had been recorded pre-electrically, or better still on a Mapleson cylinder. It isn’t quite as bad (or good) as that, but certainly there is a sense of being a kind of privileged interloper – one who tunes in to the radio from afar, or perhaps is allowed to listen from a secret place high above the stage.

Still, the privilege is real enough. Many brilliant, and some delicate, things are done, and the ‘L’altra notte’ (Sargent conducting) is vintage Callas. So, too, are the arias from a Festival Hall concert in 1959, said by Michael Roubinet in his notes to be probably ‘the first known instance of a recording by an audience member using a battery-powered tape-recorder’. Or piracy legalised.

The Paris disc confines itself to Callas’s début there, somewhat late in her career (1958), a glamorous occasion with all the bigwigs, from the President down, in attendance. Callas’s opening with her great solo from Norma wins a raucous ‘brava’ from a self-appointed cheerleader at the insecure end of ‘Casta diva’ but I imagine that not everybody was inwardly convinced at this stage that they were hearing the world’s greatest singer. It is easier to believe that in the Trovatore, and then, rather remarkably, in ‘Una voce poco fa’ where the spell really works – but, for that, it’s much better to see her, and her smile. And so it is with the performance of Act 2 of Tosca: here the video is much to be recommended in preference to the CD.

With the recordings from Milan and Athens the shopping-list comes into play again. The prayer from La vestale, despite the faint-sounding orchestra, is a fine example of the nobility of style and spirit that were at her command. Both of these concerts included Ophelia’s Mad Scene in Hamlet, sung in Italian and in complete identification with the character. The Milan performance finds her in better vocal health, yet she summons up the stamina in Athens to repeat the second half as an encore at the request of the Greek president.

Most special of all in this series is the Dallas rehearsal disc. Time lost for the listener (who cannot hear the muttered conversations between soloist and conductor) is doubtless gain for the audience: this is a down-to-work session, and, for her part, Callas gives all attention to the job in hand. It’s hard to say for sure, but my feeling is that we get from her here something extra. Work was what she liked, and here she is freed from the nagging consciousness of reputation and judgemental ears. I’ve not made the comparisons, but was strongly persuaded that I’d never heard her sing ‘Ah! Fors’è lui’ so well. Then they work at Anna Bolena and Lady Macbeth, ending with, in Italian, ‘Martern aller Arten’. There are fascinating perspectives of sound, and from time to time some really glorious singing.

The Entführung aria also features in the San Remo concert, which Callas shared with Gigli (no duets, alas). Here, she is in fuller, fresher voice, a voice which she displays to amazing effect in florid arias from Dinorah and Armida. This is perhaps doubly astonishing as we have just heard how she was singing two years earlier in Rome: the Macbeth and Nabucco arias are magnificent indeed, but the cost of such singing must surely have been equally prodigious.

These six discs are, for the most part, not re-issues or a reformatting of others (the one major overlap is with the ‘Live in Concert’ two-CD set of 1997, which included the items from Rome, San Remo and Athens). Personally, I would go for the rehearsal.