Good question, I never had one. From what I understand it is:
1. Dark, no or tamed highs, maybe just the nature of the technology at the time.
2. Tight, choked, punchy lows; some sort of high pass filtering, compression, resonance bump somewhere or maybe just coming naturally from sealed cabinets.
3. Not clean, some dirt, again maybe just because early bass amplifiers couldn’t sound clean.
I have three pedals that aim to reproduce this, Broughton Fliptop, SolidGoldFX Beta V, Xotic Bass BB. Honestly I find them frustrating much of the time. Yet I can never put them away for long. Can’t live with ´em can’t live without ´em.