Brahms and Tchaikovsky

This is a really tough question, because none of these works has truly received an audiophile-grade recording. While I don't think you should ever trade off the performance for the sound, here are some suggestions:

If you are looking for an integral set, I'd go with Pletnev on Virgin, or Gilels on EMI. Mikhail Rudy on EMI with Janssons is pretty good, but not in the same league. NONE of these recordings would be considered audiophile. For the first only, best go with the Argerich/Kondrashin on Philips, which has recently been re-mastered. Otherwise, you can try the BIS recording (always good sound) or the two Telarcs (Watts or Guiterrez).

Brahms is equally hard. Again, Telarc has a good set under Mackerras, even if his notions about using a smaller orchestra are not necessarily "authentic" or backed up by an rigorous research. Berglund (Ondine) also uses a small orchestra and it sounds excellent. Other good choices, but again, not audiophile (still sound good, though) are: Bohm (Eloquence) and Klemperer (EMI). You'll be surprised.

Hard to recommend something that is not at least a good performance. What good is great sound if the performance is mediocre?

For the Tchaikovsky try Santiago Rodriguez with the Sofia Philharmonic, conducted by Emil Tabakov. It is an Elan CD #2228. It is a 1996 recording. Also comes with the Grieg and Liszt

Libor
You make many points I also will make, don't get hung up on audiophile quality sound, all these sound good enough, seek the stellar performances.

TCHAIKOVSKY Piano 1 - must agree with Libor here, the Argerich/Phillips live recording conducted by Kondrashin is 1st choice for me, Argerich gives defining performance with tremendous energy and passion, explosive finale.

Also like the Wild/Chesky version conducted by Fistoulari. For budget price CD the Glemser/Naxos is preferred by me over many full priced versions and offers tremendous value.