Eric, welcome. I'd first suggest not getting too enthusiastic about multi-channel SACDs. Very few classical compositions(e.g., the Berlioz Requiem)have performers positioned to the side or back of the audience. The surround sound on most classical recordings is made up of the ambience reflected from the sides and back of the recording venue. I always play 2-channel source material in surround by using modes such as DPLII, Neo:6, Neural Surround, etc., to extract the ambience from other directions that had to be mixed into the front channels(there was no place else to put it)and send it to the surrounds where it belongs. They do a quite good job of this, depending on how much ambience is available to redirect, and the spacial effects of multi-channel classical SACDs may not be significantly better. Aside from that, an SACD may or may not be better in general sound quality than a CD, depending on the care that was taken in producing it.
Having said that, some suggestions in the areas that you mentioned can be made. The Mackerras set of #s38-41 that Eric linked is probably the outstanding Mozart symphony set in multi-channel SACD, but try for a better price than the regular Amazon list. There's no SACD with #25, although there's a fine Mackerras CD which includes it.
For the Mozart Horn Concertos, a good choice would be
Marriner and Civil , an original "quad" recording, so it's actual surround, not like many of the SACDs from 2 and 3 channel source material.
For the Firebird Suite the
Jansons is fine.
The Planets is one of my real favorites(I have six), but no SACD currently available is recommendable in my view. Play the Mehta or Dutoit CDs in surround.
