All my pots in the passive attenuators are 100k, and I have since rectified the problem (I think) by adding a TL072 circuit like you suggested above. Again, as my experience as well as knowledge is fairly lacking still (I prefer learning as I go along), I have run into another problem regarding the output of this set of oscillators.
Originally, I merged all the signals by joining all the ground wires of the output jack sockets, and then doing the same with the tips of the jacks, only with a 470r resistor on each to avoid clipping/distortion I presume? Anyway, I have since merged them into a TL072 circuit (here: LMNC Super Simple Mixer [Strip Board Layout + BOM]), expanding on this example, from 3 inputs all the way up to 10. Also, as I want all the inputs to be at the same volume level, I skipped out on the paotentiometers and wired the pink output lines in the diagram straight from the input jacks into the stripboard circuit.
Now those are the only changes I have made to the schematic, and it works fine, the pot I inserted before the output jack lowers the signal level without changing the pitch at all, that is perfect. The issue arises when I send signals to more than 2 of the input jacks. The moment a third is connected, all the signal cuts out completely and I have no idea why, Sam in the Super Simple Mixer post. says that in theory, I should be able to connect as many inputs as I want, so unless I have gone wrong somewhere, I don’t see that it would be an issue with the circuit? I wonder whether it may be an issue with the TL072 chip itself but it merges the first two signals with ease and so I’m finding myself fairly stuck at the moment.
As always, I really appreciate all the help or advice I can get and once again, thank you very much for all your help!