Decca has been mining their very, very deep back catalogue (now including both Philips and DGG under the Universal Music family) for some time, by converting many of their analogue recordings and early digital releases to CD at super bargain prices. I have the first analogue box of 50+ CD's, but don't have the Ashkenazy box. I also have all of the classical analogue issues (from the late 50's to the early '80's) on vinyl. For performances, the Decca analogue box is tough to beat. Ashkenazy is also a great pianist, with a wide repertoire, mostly 19th century, which I would think is captured well by his box. The original Decca engineering is IMHO the best of any of the major labels of that era. Kenneth 'Wilkie' Wilkinson is considered by many as the greatest recording engineer of all time. He did many of the recordings in the boxes. My friend, John Dunkerley, was his protege and also did many recordings starting in the late '60's, through much of the digital era. They used the famous Decca Tree arrangement of microphones to capture the sound. Decca also had many top producers and engineers, as well as boat load of top artists, many of which were extremely loyal to the label. Ashkenazy and Solti both were Decca artists for over 50 years, while Pavarotti was a Decca artist until his death. So for those reasons (and at slightly over $1 per CD), I would think the boxes are no brainers.
As far as the transfer to CD goes, I have done some comparisons of the analogue box with vinyl versions, 15ips master tape copies (Tape Project) and the best CD versions (from FIM). The CDs are distinctly inferior IMHO. Although I had interviewed Raymond McGill, who has been leading the CD box reissues for Decca, for my book, I did not ask him about the reissue process on these boxes. I am surmising that there was no remastering of the tapes (like Winston Ma of FIM had done - by Paul Stubblebine) and the reissues came from digital copies that Decca had been doing. I've seen photos of a few master tapes indicating they had been transferred to 24/96, but don't remember the dates. The digital recording were done at 48/16 using a special Decca modified recorder rather than at the standard 44/16. So I would guess these are down samples (to 44/16) for CD release. Again at $1 or so retail, with some fine artwork and a nice book with the box, I don't see that there could have been a lot of money spent on the actual processing. Certainly when I compared the Oistrakh Bruch/Hindemith album to my original vinyl, Tape Project tape, and FIM CD release, there was a distinct drop in quality when I played the Decca box version.
If you have really good playback equipment (vinyl, tape, digital), then the boxes will allow you to determine what you like and go for the much more expensive versions in other formats. However, excellent copies of early Decca vinyl (wide band grooved labels of SXL2000 series for example) are equal to the price of several boxes. For example, the Oistrakh Bruch/Hindemith can fetch around $1000 or so in near mint condition. The wonderful FIM CD's (I have all 17 of them) are around $35 per CD. The two Tape Project tapes are no longer available. Other reissue companies have also mined the Decca analogue catalogue, in vinyl in the $35 per disc range. You can find some Ashkenazy vinyl from the mid '60's and later at more reasonable prices ($5-10) per disc.
BTW, Ashkenazy (now in his '70's) is no longer playing piano much in public. He moved on to a quite successful conducting career, and we hear him conduct quite regularly in our regular visits to London, where he guest conducts the Philharmonia Orchestra.
Hope this helps, Larry