The Simmons MTX9 is one idiosyncratic piece of MIDI drum machinery. Its baffling mix of cool lo-fi percussion samples, a rather advanced MIDI implementation despite being primarily meant to be triggered via pads, and a supremely obtuse user interface makes it difficult for one to form a uniform opinion about it. For me, tolerating its less approachable sides was mainly due to its funky built-in delay but having to check the manual for what infernal button combination performs which mundane task (with the one-digit numerical display and an assortment of LEDs trying their best to provide feedback what was going on) eventually proved too cumbersome.
Simmons could have done two things differently - and completely against mid-1980s instincts - to make MTX9 into a cult secret weapon, if not a full-blown cult piece of gear a decade or two after the fact. One, they could have decided against expensive EPROMs and recreated some of the percussion instruments using analogue circuits, at which they were one of the market leaders anyway. Two, they could have changed the MTX9 form factor from a 1U 19-inch rack into a knobby desktop module, something they had also done before. However, the MTX9 was intended as an expander with its three-piece kits to be set up beforehand and then recalled by the drummer when required, so what frustrates and puzzles today made actual sense back then.
The MTX9's thirteen samples (there are two undocumented ones, a cool tom and an acoustic snare) can be radically transposed in both directions - and in real time, too! - made to respond to MIDI velocity in volume and/or pitch, which I found a quite radical but obviously welcome feature, and independently delayed before being sent to a combined mix output as well as separate outputs (which aren't sensing so plugging a jack into any of them doesn't take the corresponding sound out of the mix). The delays were definitely my favourite thing about the MTX9 - each of the three is independently configurable for rate, decay and number of repeats and the settings are storable along with all the other settings. There's no feedback (I suspect that's because the delays are created by repeating and enveloping the samples rather than using an actual delay line) but this feature still single-handedly saved the MTX9 from certain and utter obsolesce.
What is undoubtedly keeping the MTX9 from appearing on live shows, the last place where it would realistically have anything to offer, is the fact that you can't do anything quickly on it. Most functions are accessible through a combination of front-panel button presses and unless you memorise them, you can get confused very quickly, which is obviously the one thing you can't afford in a live situation. Even the build quality speaks in its favour: despite being close to 25 years old now, it has never failed me, the dials and buttons register like new, and the outputs are clean and not terribly noisy. A nice piece of kit, therefore, just not nice enough.