I never said that Tanya has more Kedusha than a Chumash, nor a Gemara, all I said is that some Chassidim won’t place any other Sefer on top of a Tanya, that includes even the Rebbes own Seforim, such as Likkutei Sichos or Maamarim. There is no Halacha that says that one MUST place certain Seforim on top of others.
Seriously, there might be some actual issues in Lubavitch, but this isn’t one of them. It doesn’t contradict any Halacha not to place a Chidushei HaRashba (for example) on top of a Tanya. Tanya was and is revered by almost all Chassidim, not just Chabad as an extremely Heilige Sefer. It’s not about generations and Achronim and Rishonim, it’s about the Sefer itself. No one has a problem placing a Ketzos on top of a Shulchan Aruch HaRav for example. Or a Likkutei Torah (another one of the Alter Rebbes Seforim).