OK. This is a fun one.
The "Haz" as it is called... it was designed by Stuart Spector in 1979. He built the first one, the second one and the third one himself. And he decided, "this is nuts." So he took the blueprints to a friend, Mr. Henry Zajac. Henry founded, owns and runs "Haz Laboratories". And his degree is in engineering with a focus in sound tech. Henry would also work with Ned Steinberger in developing the proprietary circuit for Ned's famous headless bass.
Just so that you all know, "Haz" is a simple and quick abbreviation of the two circuits made for Spector by HazLabs... but this abbreviation is not a fond one at Spector. Because you're talking about Stuart's circuit. Not one that Henry designed.
So the proper names are the Spector USA 18v and the Spector USA 9v Tone Circuits.
The design of the original 1979 circuit has changed over the years. Mainly, a few of the original components have changed by necessity as the contractor/company that made them either folded/closed or were purchased by another company and that part was changed or discontinued.
The original on-board pots that were specifically designed to fit onto a non-conductive substrate material... or the board design that Stuart developed for the Spector USA 9v Circuit.... these have changed a couple of times.
The 18v circuit is a different design, using JFET tech. It's a newer design meant to eliminate some of the inherent complications of the older, solid state 9v system. So for this little dive into the Spector circuit history ocean we're only talking about the original 9v USA design, the Kramer copy, made in both Japan (for a short period) and Korea. And the Mighty Mite (which many do not know, are the left over Spector copies from the Samick period of NS-2A construction during the last year of Kramer's operations).
OK, so here you go:
1979-1986: First Gen or Brooklyn Spector USA 9v Tone Circuit.
Used a dedicated pot system and 1/4 inch plug insert that was prone to burn-out. Also, a common problem was the battery connector would/could be broken off the board if you were not careful in how you inserted/detached the battery.
The growl and grit of the USA circuit comes from banks or rows of OpAmps which are used to handle the over-saturation issue inherent in the OpAmp design. This creates an overdriven/distortion quality to the circuit without the use of an overdrive or distortion circuit. The outcome of handling the over-sat weakness most circuits of this time suffer from produces an articulate yet growly/aggressive/gritty tone that cuts through the mix.
1986-1991: 2nd Gen or Kramer-Era Spector USA 9v Tone Circuit. Battery clips were included on the board. A new vendor for the pots was chosen. A new jack design was included.
NS-2A 9v PASSIVE CIRCUIT DESIGN: Originally built in Japan. Eventually moved to a Korean Contractor. NOT designed to power active pickups. Totally different pot system 500K pots used. Different non-conductive substrate design (BT-Epoxy VS FR-4). Staged OpAmp system used for USA circuit is reduced here. Fewer OpAmps needed in the array. The signal output from passive pickups doesn't require them.
MIGHTY MITE MM114: When Kramer was nearing the end of its life and needed to juggle vendors to keep production moving forward, they stopped using their Korean contractor for the building of the NS-2A and moved to Samick. This new circuit design was meant to replace the original design of the NS-2A circuit. When Kramer folded, a number... my sources say a couple of hundred... of the MM114 circuits were still around. Although loosely based on Stuart's design, the end result was not the same as what most Spector owners would expect. These were weaker, less growly circuits.
To the best of my knowledge, the Mighty Mite MM114 has been sold out and no more are available. Someone might want to try and order one. I've heard they're all gone. This is NOT anything close to the Spector USA 9v Circuit.
The Modern Spector 9v USA Tone Circuit - 3rd Gen and 4th Gen: There was a shortage of USA circuits a few years ago and Stuart was unsure if they were going to continue to be able to make them. The company that made the special pots that went onto the circuit board design of the Spector Circuit went out of business (or stopped making them... I'm not clear on which is the case... they just stopped being available).
No new USA Circuits could be built until a replacement pot was found. Stuart and Henry finally found a company that made a pot that would work with some modifications. The circuit was redesigned for these pots and production started again.
The last change (4th Gen) came about when EMG developed their new quick connect pickup system. Henry redesigned the posts on the circuit to quickly/easily fit stripped wires without the need to solder them.
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The changes over the years have been small, but necessary. And yes, these changes have created what equates to a tonal difference between the 1st Gen and 4th Gen boards.
In essence the design remains unchanged. The pots are different. There are small changes to some of the other components and different manufacturers have been used for the source of the OpAmp modules through the years as some makers changed their design or stopped offering certain configurations.
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The most desired (for purists) tone for the NS-2 comes from the Brooklyn models and the original Brooklyn Circuits.
The most commonly heard, most commonly sought after tone (Mike Starr, Gene Simmons (unmasked), Eddie Jackson, Doug Wimbish, Ian Hill, etc...) is the Kramer-Era models.
The most stable and best built circuits are the ones being produced today.
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It has been my experience in comparing the many different historical and modern USA Spectors that I've been very fortunate enough to have access to, that the Kramer Circuits are very strong regarding output.
I have GuitarRig5Pro set up on my computer with a set of Pre-Sets I've developed for things like Chorus, Distortion, etc... And the input levels/output levels, EQ, etc... everything is set for one of my basses. Example: Studio Chorus-The Black Pearl.
When I use these presets with the basses they are intended for, they don't clip, they don't blow levels out. etc...
Kramer Era NS-2 #1579, The Lava Bass... that bass's output was 30% hotter than the Black Pearl's. I could plug it into ANY of the presets for the Pearl and it would clip, blow out, etc...
That bass has a typical Kramer Era Spector USA 9v Circuit.
It's just that some things have changed over the years.
Today's circuits are more balanced, more tame. They have growl and depending on the application (for instance The Blue Dream's dual J pups have bark and bite that the Pearl just doesn't have) they can sound very different.
That's my whole Circuit history. A quick rundown. I didn't review my notes. This is top of my head.
I may have some of the facts wrong... I might have mixed up the substrate materials... anyway... it doesn't matter. The Korean circuits use a different substrate than the USA. LOL. It's all the same. I'm not sure that really equates to anything regarding tone.
. Bliss 
