Since Octave mode works at full volume, you can focus on the Ring-mode signal path first. The Blend pot passing clean audio at 100% dry is a useful clue.
Have you checked the signal levels around the wave shaper section? Comparing the IC pin voltages in both modes might help narrow it down.
Check the voltages around the transistor stages first. Also, verify the bias voltage at the correct test point. Half the voltage at R101 may not be the correct target. Check your transistor pinouts and solder joints, too. A wrong pinout can cause very low output.
The TL072 normally runs at higher supply voltages. Typical supplies are ±9 V or a single 18 V. At 5 V, input and output headroom become very limited. The TL072 isn't generally specified for such low voltages. In the Terrarium, 5 V may power digital circuitry. Whether the TL072 needs 5 V depends...