There have always been close parallels between dance music, and heavy metal, one in particular is the exponential rate at which new sub-genres have appeared (and disappeared) since the mid-80's.
It only takes an accent on one 16th note of a hi-hat, vs the accent being on a different 16th, in an otherwise identical beat, and dance music aficionados will passionately hail (either love or loathe) it an entirely new genre.
Likewise, with heavy metal, seemingly insignificant details (from the specific delay pedal used, gauge of strings / plectrum, colour of wallpaper in the studio reception area), folks will get passionate, and extremely heated over a sub-genre of a sub-sub-genre...
I would expect that to extend to bass amps too, so it's probably impossible to determine what is currently iconic : ie an essential component of "Metal" today.
But in terms of "iconic" in the sense of touring rigs used by iconic heavy metals bassists :
Steve Harris - Trace Elliot
Lemmy - Marshall
Cliff Burton - Mesa Boogie
Jason Newsted - Ampeg
Tom Araya - Marshall
Justin Chancellor - GK + Mesa
Throughout the 90's, I generally saw a lot more Solid State bass amps, than tube amps (mostly Trace Elliot, and GK - though also some SWR, and Hartke).
The majority seemed to favour a clear, articulate sound, from a SS amp, rather than a driven, compressed sound, from a tube amp - when trying to compete with 2 or 3 insanely loud guitarists, and a loud as hell drummer swamping the low frequencies with double kick barrages.
Other than the occasional Brit using a Marshall bass amp, it was almost entirely American bassists that would use tube amps - but even then, I'd guesstimate it was probably well under 25% that were using tube amps.
In the non-professional leagues, it was whatever was available at a good price - mostly Peavey, Carlsbro, Custom Sound, Laney, and the odd Marshall, Orange, or Fender.