Of the three sets here, it is the Philips that aims at completion. Not only does it offer the widest survey of the sonatas, including a few unfinished ones or sonata movements, but it also gives us two sets of variations. However, it does not include the Sonatas, K55–60, that Grove calls spurious and which figure in the Willi Boskovsky/Lili Kraus set, recorded before the publication of the research that changed their status. The next thing to notice about the Philips issue is that three duos are involved (and harpsichords as well as pianos) and that the recordings have different locations, with dates ranging from 1974 to 1990. While the sounds may be consistent enough to satisfy most collectors, I am less sure about the styles.
Isabelle van Keulen and Ronald Brautigam give a clean account of the music on the first disc, but their tonally unsubtle playing conveys little charm and the close recording does not help: if you can, compare their account of the 'For beginners' Sonata, K547 with the more musicianly one by Goldberg and Lupu on Decca. Most unusually for Philips, there is also a production fault: in the C major Sonata, K403, the finale begins at 2'58'' in track 9 instead of occupying track 10 as indicated in the booklet, which starts instead halfway through its thirteenth bar. Over three quarters of the 3'46'' of the C minor Adagio, K396 is for piano alone, and Mozart only wrote four bars of what remains of the music. This is one of the four unfinished works on the CD which were completed for his widow by the Abbe Maximilian Stadler. The 16 sonatas up to K31 that occupy two vigorously played discs featuring Gerard Poulet and the harpsichordist Blandine Verlet are mostly of only documentary interest (though they improve somewhat latterly), which is hardly surprising since Mozart was just ten when he wrote the last of them.
The other four Philips discs are more interesting and Arthur Grumiaux and Walter Klien offer playing with spring in its step and a lyrical yet unsentimental warmth in the slow movements, though that of K296 might with advantage have lingered a little more. Maybe there could be more charm here, but better this than prettified Mozart and, make no mistake, these are stylish artists who are also eloquent in the unusually wistful Andante con moto of K380—as are Goldberg and Lupu, for that matter. They're also deft in the vivid first movement of K377 and offer a delicate charm in its minuet-finale, while the fine but tricky late Sonata, K526 is done with panache. These early 1980s recordings of Grumiaux and Klien (in a Swiss location) have a pleasing warmth, although they sometimes favour the violin. There's a valuable booklet essay by Alec Hyatt King.
The Szymon Goldberg/Radu Lupu Decca set was recorded in London in 1974 and issued on six LPs. It is good to meet these performances again, this time on four mid-price CDs, for the playing has vivacity and warmth in the right proportions. Goldberg's tone has a touch more edge to it than that of Grumiaux, and is just as pleasing in its own way, while Lupu brings personality as well as attentive partnership to his keyboard part. There's an agreeable quiet charm in the way they handle, say, the finale of the two-movement Sonata in G major, K301, but examples of such skill and empathy abound and the finale of K526 has elegance. I have a few reservations (e.g. the finales of K303 and K377 are too romantically dreamy for minuets and the first movement of K304 is over-spacious) and feel that collectors wanting just the better known sonatas, which are also better musically, need look no further than this four-disc set.
That leaves Willi Boskovsky and Lili Kraus, in mid-1950s performances. Unfortunately, although there's little tape hiss, some have been recorded or transferred too closely for comfort. Even after a drastic lowering of the volume setting one feels that the violinist is playing into one's ear, while conversely the piano tone is muffled. Elsewhere the violin sound (e.g. as recorded in Vienna in the first movement of K296 on the second disc, track 3) is oddly stifled. The performances themselves are mainly attractive enough but they are not very varied. There is also sometimes a rather mannered, lightweight element from the pianist, as in the finales of the Sonatas, K304 and K377, which also detracts from the energy of the outer movements of K526. This six-disc set will be of interest mainly to collectors of Boskovsky's work. For others, the Decca issue will be more attractive. As for the Philips, I would mention that their fine Grumiaux/Klien performances are still available in their original four-CD full-price format.'