Watch out for mixing apples and oranges -- in this case, symmetrical vs. asymmetrical clipping refers to
how a waveform is clipped (distorted), while
germanium refers to components. Germanium could refer to the transistors (gain devices which can add distortion), or it could refer to germanium vs. silicon clipping diodes.
Text descriptions for these ideas tend to fall short, but asymmetrical clipping tends to be perceived as more "tubey" and round, while symmetrical clipping tends to be a bit more clear and crisp.
Here's a nice
commentary on the differences between germanium and silicon clipping diodes. In circuits that use clipping diodes to create signal distortion, various combos of clipping diodes can result in symmetrical or assymetrical clipping. (One of the most common mods to Tube Screamer and variant circuits is changing the clipping diode arrangement to give a different distortion flavor.)
Another TB'er might be better able to detail the differences between germanium and silicon transistors. Silicon transistors are much less sensitive to temperature variations than germaniums, so they tend to have a fairly stable sound regardless of environmental conditions, while distortion/fuzz circuits using germanium transistors can sound noticeably different at different temps. It is a fairly common perception that germanium transistors have a "squishier" and "warmer" sound when driven to distortion, while silicon transistors have a more "harsh" and "crispy" break up sound, but that tonal harshness can be compensated for with other components in a pedal circuit, so this silicon-transistors-sound-harsher perception doesn't always hold true.
For an interesting audio comparison between germanium and silicon transistors in otherwise similar circuits, go to
Fulltone's web site and compare sound clips of the '69 mkII fuzz (germanium transistors) and the '70 BC fuzz (silicon transistors).
