+100 on everything said so far, the Sorado was one of the last survivors of the original Conrad Sundholm designs. My Sorado rig has served me faithfully since April 1971, with only 2 re-tube/re-cap jobs in the interim. You're right, they are incredibly clean & bright for a bass amp, in fact I have been using a 412 Sceptre cab for bass because I found the 215 cab surprisingly "toppy", the 4 12s give me better mids overall.
I question whether the original owner really bought it in '73, by '72 Sorado & its identical siblings had been re-badged as "190B" for the heads, but that wouldn't be the first time one of us old guys misremembered exact dates from that era
But whatever, enjoy your acquisition, & welcome to the Sunn fanatic club!