To really understand what happen with the DX9, let's go back to 1986 and look at how Howard Massey described it in his seminal book
The Complete DX7 – Chapter 16: Other Digital FM Instruments
By Howard Massey
Yamaha DX9 (written in June 1986)
As new as digital FM technology is, this is an instrument which is already out of production by Yamaha and has been so since mid-1985. The primary reason for this is the original list price: nearly $1400. In other words, for only $600 more than the price of the very limited DX9, you could buy a full DX7. It was this disparity, more than anything, that contributed to the commercial downfall of this instrument. Nonetheless, it uses the same digital FM system as the DX7, but with far fewer features, and it boasts the same robust construction as is found on all Yamaha instruments.
There are still many DX9s in circulation but most seem to have been bought by people in frustration at the time they spent on the waiting list for a DX7. Once DX7s became readily available, the demand for DX9s virtually disappeared overnight! One can only speculate that this instrument might have done so much better if it had carried a lower list price.
The trouble was that the DX9 couldn't be described in terms of what it offered; it was defined by what it lacked compared to the DX7. It had no unique features compared to other DXs and buying one only made sense if you couldn't afford or get hold of a DX7. The DX9 then hit another snag, Yamaha had produced a difficult to program digital synthesizer with a limited run and small user base, something that almost guaranteed no third party support.
The DX7 had a host of third party companies programming sounds, producing cartridges and creating improved or extended firmware, something that simply made no financial sense for the DX9. With no support and limited sales, the DX9 disappeared quickly once Yamaha could produce enough DX7s to meet demand. Unfortunately the DX9's initial history has gone on to (unfairly) define it ever since, something I want to explore in "Worst FM Synth Ever"
However, before we get into that let's start with an accurate list of what the DX9 is really missing, in terms of functionality, compared to the DX7.
The DX9 keyboard is not velocity sensitive and it does not respond to velocity over MIDI.
The DX9 lacks the ability to store names with its 20 patches
There is no ability to set Operators to a fixed frequency rather than a ratio
The DX9 lacks a pitch envelope which would allow for acyclic pitch variantions
The DX9 has 4 rather than 6 operators and 8 rather than 32 algorithms
Both keyboard and MIDI do not respond to aftertouch
The DX9 has 20 patches compared to the DX7s 32
The keyboard scaling lacks the DX7's curves and break points and has simpler Rate and Scaling parameters.
The DX9 has a tape interface and no RAM/ROM slot
Slightly odd one this - the LFO has no Sync setting. There seems no good reason for this as it's all implemented in software. Perhaps Yamaha couldn't figure how to fit the parameter into the 20 buttons
The DX9 has a pitch bend range but no pitch bend step
Why the initial DX9 data tape patches didn't match their descriptions