I output my triangle to a x2 ompamp amplifier to get the output level I want, like this:
Vto comes from the 3340, TriangleOut goes to output jack.
It is easy to add something like this to the sine as well.
The opamp the circuit you posted is configured as an instrumental amplifier. It is possible to change the output level, but I think it easier to just add another amplifier.
Changing resistors can change output level. Making R13 bigger will result in a lower signal, making R14 bigger will result in a higher signal. Try to keep the resistors between 10k minimum and 500k maximum, to minimize current flow. Too high resistors will increase chance of your circuit to pick up unwanted signals.