Regarding to what makes a classical recording good, it depends a lot on your opinion.
Personally, if I am forced to choose between a classic, wonderfuly performed piece with bad sound enginnering, or something less amazing with excellent sound engineering, I usually go for the last.
This shocks many people, but I have no problems trading a fair amount of performance quality for good sound.
I have realized I seldom listen to classic amazing performances... for instance, Callas' Traviata recorded here in Lisbon...critics die for it, yet, that Mono sound gives me the creeps, no matter how wonderful Callas and Kraus sing. I'd prefer to hear any young, inexperienced soprano singing at some cheap unknown label with top-notch sound quality.
So the trick is... try to get both! I usually track down a few versions of the titles I want (choosing by performance quality and reviews)... operas ,whatever.. and from those, then select them based on the sound engineering quality.
For La Mer, you can go the cheap way with Naxos, with good quality.
PRELUDE A L’APRES-MIDI D’UN FAUNE + NOCTURNES + LA MER
-BRT Philarmonic Orchestra, Brussels (Alexander Rahbari)
-Ed. Naxos, Rec. 1989, DDD
I am quite pleased with this recording so far, even after listening to an excellent La Mer played live.
And for the Firebird... again, why not save some money...This is also a very cheap CD...
-THE RITE OF SPRING + THE FIREBIRD BALLET SUITE 1945
-Royal Philharmonic Orchestra (Yuri Simonov )
-Ed. Royal Philharmonic Collection /Tring, Rec. 1997, DDD
Sound quality is excellent, and the performance is also pretty good (Check Grammophone)