The opamps are 6dB gain fixed (see Burr Brown's datasheet on the INA2137) but you can tweak this yourself; really, the use of this opamp is a hang-over from the Mk1's diff board which cleaned up the audio outputs. Note that a meter may well be showing you the RMS value, the full swing is almost 4v on a scope with a 0dB 1kHz sine ISTR.

They're fed from a 10v rail, so there is room to get the swing up but ISTR it won't just be a component change, it'll be a lifting pins job. We have plenty of spare INA2137's if you make a mistake, though

Hugo