Super helpful, and yet still very confusing - thanks for sharing this! It does look like it's boost-only, but the way the two band work together isn't simple/predictible.
I guess that's why the sound is so unique. I'm still amazed by the quality and versatility of this bass, especially for the price I got it for!
I'm most certainly not an expert on the MM SR, but do nerd out on the inner workings of instruments.
In a simplified nutshell...
The 2EQ preamp was designed for a musical instrument tonality, not an audiophile goal of 'flatness.' The preamp is designed to get a variety of nice, usable
bass guitar tones. The preamp isn't really capable of 'flat,' the amp it's plugged into is also not capable of 'flat.' Flat was for PA or audio.
When the SR was being designed, it was to have active electronics, as that was more modern and forward thinking vs the 'old' passive basses of Leo's history. While in prototyping and beta testing, it helped to have the pickup moved further towards the bridge to make room for the slapping/popping that was coming around. With that pickup placement, the instrument lost some low end, which was made up for with the extra bass from the preamp. Also, the pickup and preamp brings on some really sizzly highs, which was mostly lost on the bass cabs of the mid 70s-early 80s, before horns and tweeters became the norm.
Therefore, the preamp needed to be able to really boost some lows, but have somewhat typical boost/cut in the treble. That's why the bass control barely cuts at 0-1, but then goes into boost from about 2 and up, and the treble control is cut from about 0-4 and boosts from 5 and up.