you have a couple of choices, all of which require some circuit analysis and the use
of a soldering iron. if that's OK with you:
1. adjust the volume control. its usually a log taper, so measure the pot's resistance at the
maximum volume setting you'd like. this is from the input signal to the wiper. then replace
the pot with a value that is the difference from the original pots maximum resistance to that
maximum resistance you just measured. then add to the top of the new pot the new
resistance you measured. not perfect but it will be a good start. this leaves the loading alone
2. use the tone controls (not sure it can or cannot be defeated or set to unity gain) and
lower the gain at the opamps. usual formula that sets the gain with two resistors - just replace
3. use a resistor network at the output to lower the output level. start with pot and adjust it
to taste, then replace with two resistors that match the pot settings.
4. easiest: lower the outputs of your sources. if more than more use a linestage with an
output control and adjust accordingly - no design change, no soldering. just an extra
component
5. remove fx-audio. or lower its output volume.
post your results - I am sure you're not the only one with this issue.