The circuit before this amp, biases the triangle waveform around 1.6 volts. By just changing any resistor you will probably upset the bias point. Then the waveform will be too close to ground or +12V. The gain has to do with the ratio of the emitter and collector resistors.
The pullup is done by a transistor so it has low impedance.
The pull down is done only by a 10k resistor. You could use a 1k resistor here.