These are 55Hz resonance speakers, hence the "Bass Series" badge but sound great with guitar as well as bass.
These amps were more or less a copy of the Fender 1959 Tweed Bassman. This was the combo version of the 2098 Master Lead head. The 2000 Lead and 2001 Bass heads are probably the loudest and most powerfull Marshall tube amps ever built.
Their first amps were very heavily "inspired" on the 1959 Fender Tweed 4x10" Bassman. In the brochures they were called Artist, but the panels on the amps show Artiste. The head amp was also available seperately as model 2048. The schematic is the same as that for the 2040 Artiste amp.
He did this together with technical friend Ken Bran. It is said that these amps were only available through mail order. The Artiste speaker cabinets were in production from 1973 to 1977. I've seen these combo's fitted with Celestion G12H30 speakers. This two piece setup, the head and cabinet, were sold together as Model 2041.
Desperate to build a factory large enough to meet demand, Jim Marshall made a deal with the Rose Morris company. (Thanks to Christos) This amp was in production from 1976 up to 1980. It was fitted with two Celestion G12L T1632 8Ω speakers. Special high power version of the speaker cabinets were made for these beasts. This was the accompanying cabinet for the 2001 Bass amp.
In exchange for the necessary funding (to build this new factory) Rose Morris acquired the exclusive worldwide rights to distribute Marshall amps for the next 15 years. See the Marshall Amps 2000 page for more "official" information on the 2000 amps.