Not to be obvious, but the 'B' models were offered as slight model changes and improvements in design became possible for a given price point. So you have a 2220 and a 2220B, etc. Both may have been offered at the same time until inventories of the earlier model were depleted.
Inside the cover of a 2230 receiver the mounting brackets that hold the dial lamp assembly are made of metal. In the 2230B they are plastic and are known to develop cracks as the plastic parts age. The 2235 has a single tuning meter, the 2235B has both a signal strength meter and a center channel tuning meter. I believe the model 2220 has no midrange tone control, the 2220B has the midrange control.
To compete with other brands, Marantz was forced to provide additional features for the same price as before. These examples give some insight into what Marantz was doing while trying to save money, and/or keep the price down.
During the late 70s the U.S. experienced a large drop in value of the Dollar against the Yen, more than 30% in just a few years. As the U.S. was the major market for these audio products, Japanese manufacturers had to scramble to make their products cheaper to build (more integrated circuits and automated assembly) and cheaper to ship (less steel and aluminum and more plastic, less weight), if their U.S. customers were going to be able to continue to afford them.
In the later models, physically smaller power supplies were used because they cost less and weighed less. They were still adequate to the task, whereas the earlier ones may have been oversized for better stability and performance.
For headphone use, any of the 22xx receivers in the 15 to 40 watt range are an excellent choice, there is no need for one with more than this amount of power as any dynamic headphone * will use far less than 1 watt.
* The AKG K1000, unique among dynamic headphones, requires an amplifier that can deliver several watts of power at 120 ohms for full output. As owners ofthe AKG K1000 have learned, the Marantz receivers can drive this headphone.
