Given how iconic the B15 is.. and I think I know the answer to this already, but hey..
How come there aren't many cheaper smaller wattage all valve heads out there?
A quick eBay trawl will list many many guitar valve heads under £300 or so - but for Bass.. nope.
Now I know that for live use those 30-100w will be, er, less than ideal, but I don't get why more valve heads weren't made in the 70's, 80's 90's and up to today to fit into that "at home or in the studio" brief..