I fully support the Conte's wise comments. A few brief comments of my own:
Dutchman. One Kna recording has been preserved (Bayreuth 1955). Same cast as Keilberth's Decca recording--quite possibly, on balance, the finest cast ever assembled in this work (Uhde, Varnay, Weber, Windgassen, Traxel). Kna's conducting is generally felt to be more vibrant than Keilberth's, and Varnay was certainly in better voice for Kna. Excellent mono sound, esp in the Orfeo release.
Tannhäuser. I don't think any Kna performance has been preserved.
Lohengrin. A 1963 Kna performance (Bavarian State Opera) has been issued by Orfeo. The title role isn't ideally cast (Hopf), but the other leads are strong (Bjoner, Varnay, Metternich). Mainly of interest to those who want to hear Knappertsbusch in every possible opera!
Tristan. A 1950 Kna performance is available on several labels (best sound that I've heard is on Orfeo). This is a real sleeper. The cast doesn't look over-enticing (Treptow & Braun, with her real-life husband Frantz as Marke!) but they are all in surprisingly good form and, as a result, the performance is commonly judged one of the very finest
Tristans on record (see, e.g., the discussion in
Opera on Record, ed. Alan Blyth, which names Treptow as one of the 3 best Tristans on record, and Braun as one of the 3 best Isoldes). I wouldn't choose this as the only Tristan for my shelves; but if I wanted to keep only 4 or 5 Tristans, this would definitely be one of them (alongside Beecham, Furtwängler, Böhm...).
Meistersinger. I personally support 100% the Conte's praise of the 1960 Bayreuth performance, but I'm aware that not everyone shares my love of Greindl in the central role! A less controversial recommendation here would be the classic 1950-1951 Decca studio recording with Schöffler (who, as Jeeves says about Shakespeare, "has given general satisfaction") in the role. Typical fine early 50s Decca mono sound quality.
Ring. Three complete Kna Rings have been preserved (Bayreuth 1956-1958). Overall 1956 has the finest cast, overall perhaps as good as any ever recorded in a complete Ring (Hotter, Varnay, Windgassen, Neidlinger throughout in the 4 main roles). Excellent sound in Orfeo's transfer.
The 1957 cast is decidedly inferior in
Siegfried (Aldenhoff replaced Windgassen that night), and the 1958 cast is somewhat inferior in the three parts where Alberich appears (Andersson instead of Neidlinger). However, I strongly admire the 1958 Walküre (with Vickers & Rysanek as the twins) and the 1957 Götterdämmerung (with Uhde & Grümmer incomparable as brother & sister). When I play Kna's Ring, I usually choose those two, going to 1956 for the other two parts. The Music & Arts transfer of 1957 has excellent full mono sound; Walhall's 1958 is serviceable & undistracting, but probably not the best that could be achieved with this material.
Parsifal. Again I agree with the Conte about the excellence of Kna's 1952 and 1964 recordings. 1952 has surely the most detailed & nuanced all-round Kundry on record (Modl) with very strong support (esp from Windgassen); 1964 has Vickers & Hotter superb in the two main male roles. However you already have the excellent 1951 set and therefore may not want 1952 as well (the casts are virtually identical). I would also strongly recommend the 1963 recording, with Windgassen, Hotter, and a better Kundry than 1964 (Dalis), but it hasn't yet had a decent commercial release. (Afterthought: I believe Opera Depot may offer a very serviceable CD transfer of 1963.)
To summarize: Knappertsbusch & Keilberth were the 2 favored conductors at Bayreuth during the peak phase of postwar Wagner singing, so
on average their recorded performances tend to have more evenly high-quality casts than other conductors'. Of the two, Kna was of course much the more vibrant conductor. The original masters of nearly all live Bayreuth recordings of the period have been well preserved and are in fuller, more detailed sound than one might expect; in particular, the beautiful
acoustics of the house are well caught.