Hi! I think the BlĂĽthner design settled on the present-day design, or largely the present-day design around 1990. That is, with the aliquot system only on the upper-most section of the plate and no additional aliquot bridge on the lower octaves. I can tell you about why that changed if you're interested but the main point is if you get a BlĂĽthner from the 80s with an extra aliquot bridge, it's based on the old Style-VIII design. Really the only difference between the Style-VIII and the new model 6 is that the new one is a little more powerful, and it has the extra bridge.
The reason people are experiencing wildly conflicting results with the BlĂĽthner action is voicing and preparation. There was a time when BlĂĽthners were not preparing the pianos very well in the factory. That period is over. They were coming out with a general regulation, and a general voicing but the hammers were too hard. This made them difficult to control. The reason for this approach was basically to make the pianos cheaper for the dealers, and then the dealers can do the preparation. Unfortunately the dealers didn't always do the preparation.
I don't know how the geometry differs from other tier one pianos. Usually there's a slightly shallower key dip and slightly lighter touch although I'm not sure if the latest models vary from this. Yes, it's possible to make these pianos as responsive as any other piano, and no the shorter key stick isn't going to cause you any problems. Many pianists and piano professors have smaller pianos than a BlĂĽthner Model 6 at home.
Tonally speaking, the Model 6 is actually BlĂĽthner's best selling grand. People often prefer the Model 6 to the Model 4, feeling that the 6 is better balanced. I think it's probably the case that in general it is better, although some people prefer the kind of brighter sound of the 4. Of course, it varies from piano to piano and with high-end pianos it's often the case that there's more variance between them than there are with more mass-produced models. A lot of people when they hear recordings of the Model 6 think it's a larger piano, so I don't think there's going to be any problem with the size. A lot of recording studios and smaller concert venues in Europe use the model 6, and I've even seen them used in concertos although it's more effective with small chamber orchestras than it would be with say, playing Tchaikovsky 1.
My advice to you with the Model 6 is to not make it bright. Get the technician to mellow it out. Often, there's a paradox with BlĂĽthner pianos in that when it is bright you get the punchiness but it isn't projecting or sustaining. It needs to sing, and it likes to sing with a mellow sound. Of course it can be on a spectrum, but if you try to make it sound like a Yamaha C7 prepared for pop music, you'll find the piano sounds ugly. The C7 doesn't sound ugly when prepared like that because of its design although it wouldn't be right for chamber music at that point.
The Steinway B is for sure louder than the BlĂĽthner Model 6, but you don't need masses of volume in the home studio. You need balance and control. Anyway, I hope what I say helps you decide whether or not you want one, but there's no substitute for playing the piano. With older pianos condition is everything. A 30 year old piano from a top maker is still a 30 year old piano so you have to satisfy yourself that the piano is in good condition.