Why does the collector feedback resistance proportionally affect the amplitude accross the circuit?
The resistive chain R7-R8-R9 provides negative feedback for Q3 - for DC (bias point stabilization) as well as for ac. This is because of the finite output resistance of the Q2 stage.
Hence, any modification of this feedback chain will influence the gain of the final stage.

(I think, the "virtual ground" principle does not apply here. It is valid only for opamps with a (nearly) infinite gain).