I've got one of those 40W heads. Got it off ebay and it's kind of busted up. The power amp came loose during shipping. I never turned it on, future restoration project. I had a 60W head with two cabinets around 1970. One was a 1x15 (D130) and the other was a 1x12. I thought it was a 1x15 when I bought it but was surprised to see it had a D120 when I opened it up. I used it for guitar and it kicked butt. Anyone who says they never made a 1x12 is wrong. I traded the whole thing for a bass amp around 1971.
I've always thought those cabinets were tuned too high to work well with a bass guitar. They sound good only up to about half the power rating of the speaker. More power than that and they fart-out (exceed Xmax).
IMHO all bass guitar cabinets should be tuned between 35 and 40Hz which usually results in some low bass rolloff but the cone remains loaded at those frequencies and won't fart-out as soon as a cabinet tuned for flat response.
There is always a tradeoff between efficiency, bass extention and box size. The JBL D140 is not a good match to a GK400RB unless you've got 6 or 8 of them. Modern speakers don't have the extreme efficiency of the JBL but can take more power so they are better suited to the GK.
