I have an old radio ITT Schaub-Lorenz Touring International 103 which had small issue with bass control circuit. Issue was caused by "cold joint" around the potentiometer used for bass regulation.
Here are steps of the radio disassembly:
1. Remove the knobs from top panel.
2. Unscrew all screws from rear panel.
3. Unscrew all screws from top panel.
4. Remove rear panel.
5. Unscrew two screws located near the handle.
6. Remove front panel. Be careful because speakers are connected with wires to main board.
After short analysis of part of schematic related to tone control circuit and quick measurements of continuity I have found that issue was caused by "cold joint". There was no connection between of R334 potentiometer and C388 capacitor. The "cold joint" was marked by red square on one of following pictures.
After re-soldering of this joint, the issue was fixed. Here is short video of disassembled radio.