there are extensive threads on the both the suck switch and the sweet spot/bridge position Bisonic. as well as the 70s Guild bass Humbucker and deep hard switch. some brief points and thoughts, based on what ive learned, as a non-expert on all levels:
-the sixties "suck switch" circuit included a "transformer/choke/something(?)" located under the neck pickup, and some additional capacitors near the pots. the transformer/choke is the part that gets bypassed to get rid of the "suck effect." beyond that, there is likely variation in what people have done, or not done, to the rest of the circuit. the "switch" itself (the button on the bass) in many cases is likely left wired up, with those extra caps in the line, and may still do "something" to the tone, especially if desired, and arranged in a way to accomplish such (ie: abandon the transformer then convert the switch to a "bright switch" which has been detailed on this forum). with that said, some people may choose to remove all of the extra caps and just wire it like a "pre-suck switch model" SF Bass altogether. my feeling is that most people simply bypass the transformer, and leave the rest alone. that appears to be what was done to mine. as such, the switch changes the tone a little when engaged, but does not "suck." without the switch engaged it is full blast, the best sounding SF Bass ive played yet, or at least equal to other 'besties.'
the 70s DH switch performs its function well. there is no transformer/choke etc.. it does not suck anyhing, just clips singnal. when dialed in, especially the amp, the 70s humbucker Guild bass can be made to sound quite good, in either position. the Guild bass humbucker has gotten mixed reviews online, but between the vintage Starfires, M-85 solids, and JS's, I think over time they have proven their value and tone to those that enjoy playing them. i know some professionals that have owned/played Starfires both with Bisonic and Humbuckers (some of u may be reading this?) and in some cases the Bisonic is just channelling too much background noise (cell phone signals, dimmer switches etc...) and the Humbucker bass becomes mandatory. if the DH switch has one flaw, it is the location on the bass, it can be switched up or down rather unintentionally while playing if youre not careful. Apples to apples though, the Bisonic has a growl and a defined "thump" that differs from the Humbucker's sound.
-there has been discussion as to the location of a "sweet spot" Bisonic vs the bridge position Bisonic of a SF-II bass. in short, in most cases the sweet spot position of a SF-I bass is a little closer to the neck than a bridge Bisonic on a SF-II bass. But, we have found some SF-II basses have the bridge pickup closer to, or possibly in, the sweet spot position. we are really splitting hairs here and talking about 1/8" in either direction. there may have simply been variation in tooling or building styles, and/or some SF-IIs may have strated life at the factory as a SF-I, converted to a SF-II prior to leaving the factory. a while back twocorgis and I each had our SF-II Bisonic basses together. mine had the bridge pickup 1/8", or possibly only 1/16"-that was probably it- closer to the neck than his. there is also a picture of Phil Lesh's first SF-II bass, with no suck switch, and his bridge pickup in the same spot as mine. But most of them appear to be like twocorgis.'
-where your fingers are located on the strings. has a huge impact imo. a SF-I with a neck position Bisonic, when played over the brigde pickup position area, will yeild more bridge-like tone. With a SF-II, the higher notes will be more clear and treble-y when played with the Bridge pickup on. Guild made a few JS-II and M-85-II basses in 1970 that had the Bisonic in the neck and a (Hagstrom) mini humbucker in the bridge, which is super cool. id say ultimately equal to (but different) than 2 Bisonics, and some would even prefer the one humbucker option for occasional noise canceling connivence (as eluded to above).
-Maple vs. Mahogany. Maple has more of a "pop." Mahogany has more of a "boom." they definitely sound a little different. but again individual play style, bass and amp settings, and the location of your hand and how close it is to the bridge, will have the most impact.
-90s Guild SF-II bass. has different (1990s) Guild/Fender/(Duncan?) humbuckers. they sound quite good! the body of the bass is also 1/4" skinnier than the vintage or Newark Street models. They also have upgraded tuners, and a very deluxe Guild hard case
i have been writing this all day in between some busy work assignments, have not read the prior posts in a few hours. if something has been repeated I apologize.