Line-level
Impedance bridging
The receiving input should look ≥10× bigger than the sending output.
Simple
The device receiving the signal should "look" much bigger (electrically) than the one sending it — at least 10× bigger. When it does, the signal passes through cleanly. When it doesn't, you lose bass and the tone shifts.
The theory
This is voltage bridging, the standard for line-level audio: a low source output impedance feeding a high input impedance. The two form a voltage divider, so the fraction of signal delivered is Zin / (Zin + Zout). At a 10:1 ratio you keep ~91% (−0.8 dB) and, crucially, it stays flat across frequency. The engine passes ≥10×, warns at ≥5×, and fails below — because a poor ratio interacts with frequency-dependent impedances to bend the response.
For experts
The reason a bad ratio is tonal and not just quieter is that output impedance is rarely flat — it often rises at the frequency extremes (coupling-capacitor reactance at the bottom, especially in tube gear with output transformers or single-cap-coupled outputs). Feed that into a too-low input impedance and the divider attenuates more where Zout peaks, producing audible bass rolloff and a midrange tilt. Vintage and tube preamps are the usual offenders — a 2 kΩ output staring into a 10 kΩ input is only 5×, and the bass goes soft. The 10× rule is the cheap insurance that keeps the divider frequency-independent. (Power transfer is the opposite goal — matched impedances — but you never want that for line level; matching halves your voltage and doubles your noise susceptibility.)
The bridging divider: at a 10:1 ratio the loss is only −0.83 dB and stays flat with frequency.