I am a former customer service technician for H/K. I was with H/K from January 1974 to October 1988. I'm very familiar with the 900+ receiver. For about 12 years I was the only technician working on all multi-channel receivers at H/K's in-house customer service department first in Plainview, then Woodbury, NY. I know all of the weaknesses of all those "plus" models. I don't know exactly how many I worked on in total, but it had to be in the high hundreds, if not thousands. In other words, I probably had my hands on more 900+ units than any other individual technician in the world.
The 900+ (and the 100+ & 150+) was designed by H/K in the USA but custom manufactured exclusively for H/K by a company named "Crown Radio Company" in Japan, not to be confused with a similarly named American company based in Indiana. I understand that the 900+ had appeared a few years later in some foreign countries (South America?) as a counterfeit after it was discontinued by H/K, perhaps relabeled. I'd be curious seeing one relabeled... could be a collectors item.
The 900+ was basically an upgraded 150+ with slightly larger power transformers (TS-464 & TS-465 replacing the TS-436 & TS-436A transformers), a multi-color dial scale (replacing the green), and a CD-4 decoder built in replacing the phono preamp. On the 900+, the CD-4 board has the magnetic phono preamp stage. Also, the 900+ came standard with a wooden top (and side) cover. The 150+ had an optional wooden cabinet but came standard with a metal top cover.
In quad mode, each channel in the 900+ could easily deliver 45 Watts per channel into 8 ohms. In stereo bridge mode, it was rated at 90 Watts a channel in 8 ohms but easily did more. Over 100 Watts per channel stereo into 8 ohms was typical. Power output ratings on H/K products were always at the time very conservative.
Each of the 4 amplifier modules were the same circuit design (but not the same components, however) as the then top-of-the-line Citation 12 amplifier. However, the Citation 12 was produced in Plainview, NY with US-sourced components.
The 900+ was plagued with noise (hiss) at minimum volume as the 2SC458 tone board transistors aged. In customer service, I later replaced all 2SC458s on the tone board with Motorola substitutes (with a 43025972 H/K in-house part number) to prevent recurrence of the noise. Also, the tone board was a double-sided board with feedthroughs from one side to the other. Those feedthroughs were notorious for failing, or becoming intermittent, requiring re-soldering. I used to re-solder all as a precautionary measure. Gaining access to anything, especially the tone board, was tricky if you were unfamiliar with it's dis-assembly. Repairing the output boards was mostly impossible without removal. I used to remove defective output boards and rebuild them outside the receiver. I had a stash of rebuilt boards to replace the defective ones and then devoted a couple days to rebuilding a bunch so I could just replace a defective board with a rebuilt board assembly. Brand new output board assemblies were never available. Each output board was identical but had different lead lengths and therefore were coded with numbers 1,2,3 & 4 to indicate installation location.
As for the joystick, if everything is working correctly in the 900+, each channel will be equal in level and the joystick would be fine in dead-center. I thought the joystick idea to be quite ingenious. I don't know why anyone would have a problem with it.
I'm not sure anyone would be interested in any of this information but it's been stored in my brain for many years and I'm happy to share rather than taking it to my grave, for anyone interested. I wonder how many 900+ receivers still exist on this side of a landfill.
As for the 75+ and 800+, they were also designed by the H/K engineering team in the USA, but were custom manufactured by another company in Japan, not Crown.
I currently own a 75+, an 800+, and a 900+, none are currently being used and not for sale. All were acquired by me long after leaving H/K.
By the way, the 900+ pictured above has the incorrect tuning knob. The 900+ tuning knob was black. It looks like the one pictured came from a model 150+.