The original SB-40 came out for 1981 and this SB-40EQ came out only in 1982. They are often referred to as the Holy Grail of Washburns and both are quite rare. As I luckily own 2 SB-40's and the SB-40EQ you see here, I was able to take them both to an electrical engineer to get some insight on them.
The only 2 differences are:
1) 1982 SB-40EQ has blackened aluminum cavity covers while the preceding 1981 SB-40 has natural color (silver-gray), brushed aluminum cavity covers.
2) 1982 SB-40EQ bridge pickup uses only 2 wires which I understand to be the normal thing. Evidently, the 1982 SB-40EQ DiMarzio bridge pickup was slightly different from the 1981 SB-40 bridge pickup, but not to to any degree necessarily changing the tonal output. While the 1981 SB-40 bridge pickup has 3 wires (so evidently the shielded ground wire is included with the pickup and not separate and attached to one of the EQ pots).
a) This new electrical/tonal package of the SB's was so good that it was used on all Force 40's and the B-40EQ/EQW basses for the next 4 years (1981-1985). (Prior to the SB's, the electrical package was different. I'll not go into detail about those here.)
b) It can be used for any genre and really takes well to effects, keeping the bass notes clean WITHIN the effect signal instead of absorbing, so to speak, the effect which of course makes you lose clarity and brings mud.
c) The necks on SB's, again, used on all basses following until 1985, are tremendously easy to play on, in fact, to me, they are the best necks I have played on. And of course, rock solid balance in any position you put the bass.
d) the finishes are a SUPER DURABLE polyurethane, including back of neck, yet so dang smooth it does not interfere or "grab" your hand. Just wonderful finish. Mine have years of playing and show no wear at all!
e) The two back parametric EQ pots do sweep across a spectrum and you are more or less choosing freqs to "focus on", not just scooping and cutting out large swaths of frequency. They also are not each pickup-specific so much as they will affect the entire signal leaving the bass made up from both pickups or any one of the separate pickups.
f) Yes indeed the mini-switch adds a "mini-focus" and boost at a specific and narrow freq range to add "brilliance" at around the 850Hz-1000 range. I used to think it went lower but do not think that now. Some mistakenly think it is a mini-phaser---it is not.
The good thing about vintage Washburns is that they seem to be un-recognized for the great basses that they are. This means you can get them (if you can find one) at comparatively low prices!
Washburn SB-40EQ Bass
1982 only , made in the famous Matsumoku factory in Japan.