I'll try to explain the tone difference as they all play pretty much the same....awesome.
The Forte is very heavy in the low mid area. That's where its growl is centered. It's a very BIG sounding bass live. It seems a little shy in the top end but that might just be due to the fact it's huge in the low mid. When you roll the treble up it can get bright but after a certain point will become harsh and brittle.
The 1987 Kramer (orange flame) has a warm and round tone with the tone controls in the detents. It sounds a lot like the Forte there but the magic happens when you start to bring that treble knob out of the detents and roll it forward. It seems to not only bring the treble up but also brings some high mids with it all the while staying warm. It doesn't get brittle even with the treble run wide open. It is simply an amazing sound that we all have known to love (think Eddie Jackson).
The 1990 Kramer (Precious). Well, what can I say, this one is just flat nasty. It is a pissed off bass that doesn't really want to conform. With the tone controls in the detents it's bright and aggro. Again, with the treble knob, bringing it forward just gets the tone more and more pissed off. Like the other Kramer though it does it is a warm way. The Forte (and my other Woodstock NS 2's) just cannot pull this off the way the Kramer's do. This bass is, without a doubt, easily the most aggressive bass I've ever played.
I think all three will sound fairly similar recorded though. I'm no recording genius like some on here and can't get them to sound much different. Like Omer mentioned, you really can hear the differences live through an amp....especially a BIG one. It is very apparent and easy to hear the different personalities of all three. I love them each for what they are.
On the new vs. old US 9V. I've been doing a lot of A/b'ing the past few days and to me (IMO, IME, etc...) the older circuits just seem to have a touch more distortion to them. This gives my ears the impression of warmth. The newer circuit is no slouch and there are obvious trade-offs. The new circuit seems more bright and articulate but that circuit has articulation to spare.
Again, I'm not trying to sound like an expert or anything. These are just my quick observations of the basses. I will say though there is something "magic" about these Kramer's. Placebo? Maybe, but I don't care...it flips the switch inside me.
Here's a very bushleague soundclip I made last night of the blue Kramer through Amplitude. Believe it or not this is just with a touch of overdrive from Amplitudes virtual pedals. I'd say about 25% drive mixed 50/50 clean/distorted. It was in the last thread right before it got closed so some may have missed it.
http://soundcloud.com/husky123/kramer-spector-blues