I too, had this set originally on vinyl and also saw #6 at the Festival Hall, London.
The set is almost Brucknerian in its splendour; quite different and much better than the usual suspects. By now all the labels knew how to record at the various locations and these are EMI at its best. Natural balance, image, perspective, allowing full appreciation on the performance. (ie. before the technology took over).
I think this set got poo-poo'd initially because of a "Stick to the cello, mate!" attitude. If it were labelled someone else, this would be one of the top sets; it seems Rostropovich was not taken seriously as a conductor, a bit like Ashkenazy, whose short set for Decca (4-6, Manfred) is also excellent, if more conventional.
Even if you've got these by others, this Rostropovich set is hardly going to break the bank, is it? Try it.