To quote the manual for the 876 (which I in handy PDF format)...

"Leave 20 cm (8") of free space at the top and sides
and 10 cm (4") at the rear. The rear edge of the shelf
or board above the apparatus shall be set 10 cm (4")
away from the rear panel or wall, creating a fluelike
gap for warm air to escape."

Going by the book, you're space is too confined.

See the first section "Important Safety Instructions" in the 875's manual to see if it differs.

M80v2 | VP150v2 | QS8v2
SVS Pci+ 20-39
Emotiva UMC-1 & LPA-1
M22ti + T-Amp, in the Office