That was not my experience. I think it depends on the speakers you use with it. The rig was originally designed when PA support was not common and the bass rig had to fill the room on its own and was so designed.
It was sold as a matched rig, the 140 and the 106 cab, then called the 146 together. I still have the original rig and there is no lack of bottom.
Like you Eric, I have a rig with 2x12 Em Delta 12 LFA's and I wouldn't pair that with this. I pair that with an Eden Wt-400.
I played a Gibson Eb3 through it originally (with the famous Mudbucker) and there was way too much bottom, I had to use the bridge pickup only. Lately I play the rig through a Geddy Lee Jazz bass with Seymour Duncan Antiquity II's and the E string response is such that I have to EQ a little bottom out so it is even across the whole note range.
Basically, your choice of speaker cabs is more important than anything.
Why so loud? Well, it was supposed to be loud, it was designed that way to meet the needs of the times. Also, the amp was very conservativley rated and today surely someone would claim 300 watts or say sounds as loud as a 400 watt amp because it is. I never measured the SPL, but I kept up with 2 very loud Les Pauls and Marshall's at the time. It was lud enough to pull very nice sustain and feedback from the rig with the Eb3. You won't need more juice than that