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A square wave generator with a 2nd order low-pass is not actually what I would name a sine generator, but anyway. The square wave amplitude is always equal to power supply (minus residual voltages) so it can't be changed by design. Using an attenuator, e.g. R18, is the only way. Of course, the magnitude can be also attenuated before the filter. Why don't you simply adjust the output voltage by R18?