Last year I was so blown away by the Mel 9 that I wasn't sure that I could fairly review the next entrants in the series. Once I heard the choirs in the Mel 9 I knew that everything after that might be a letdown. But at the time, I KNEW this box was next, and now here it is.
I don't know what the core technology is behind these ____9 boxes, but they have some common limiting characteristics. They tend to be very compressed sounding and not terribly dynamic. They can be very slightly off/sour on sustained notes, especially on the tail end of notes. Although they can be quite trebley, they don't have the high frequency components of the original products they model. I'm going to guess that the actual high-end cutoff might be 8 - 10khz. Elantric has pointed out that they often have a slight latency on notes, but this has never been an issue for me.
With these clear limitations acknowledged, they really are modern wonder boxes. They continue to improve their tracking, with the Mel 9 being the first in the series (the Key 9 used a different approach) to be able to adequately process complex chords enough to make it usable polyphonically. In this respect they blow Roland/Boss products off the track. The SY-300 cannot in any way hang with these new boxes in poly pitch-shifting. They aren't perfect though, as a humble Maj 7th chord quickly reveals. Keep working on it, EHX.
But, these simple boxes bring gigable versions of Mellotrons and now a few vintage synths to your pedal board in ways that guitar players would have worshiped a few decades ago.
Now let's dig into the Synth 9. To begin with, this pedal is worthwhile right now if your goal is to blend in synth bass voices into your rig. That is what the Synth 9 does best of all. Most of the voices have knee-bending and speaker cone popping bass that you can adjust with good flexibility across a number of patches. I'm serious about your speakers, though. Many guitar amps aren't built to withstand what this pedal can dish out, so you have to know that this pedal may need to be in a channel with the bass reduced if you are using it in a guitar amp.
Beyond the many good synth bass settings, you get 9 fundamental patches to select from. EHX rightly selected patches that scream "Look, I'm playing a real synth! Honest!" by throwing in big filter and modulation sweeps on many of the voices. I get this, because many guitar players will use this to grab the occasional solo or chord backing that in some respects needs to be somewhat of a caricature of the original sound. These voices do indeed stand out and yell "IT'S NOT A GUITAR IT'S A SYNTHESIZER".
That said, I want to commend EHX for possibly the most intelligent Ctrl knob layouts of the ____ 9 series. What I was afraid of was that the patches would be over the top with little adjustment provisions, and that just isn't the case. Each patch actually has 2 -3 core sounds that you can find by twiddling the two knobs. In a real sense, that increases the number of actual voices to perhaps 20 - 30 distinct sounds. Great job, EHX. So, given that I personally like to blend synth sounds in with other sounds, there is enough versatility here for me to get sounds that I like. That was my personal acid test for this box, and it passed.
Now, a few comments on the individual patches. OBX is big and bold no matter how you try to adjust it, just go with it. Profit V wasn't my favorite for the core sound, possibly because the original synth is so wonderful that this one patch couldn't possibly express it well. BUT, there is a hidden soft bass in this patch with adjustments that I will use constantly. Vibe Synth is an interesting quirky synth to throw in here and there. The Mini Mood is possibly the most successful voice in the pedal, as your audience will BELIEVE it when you take that big solo with this setting. EHX Mini is better than I expected and has a few settings that I will explore further. Solo Synth is terrible useless distortion, I hate it. Maybe you won't. Mood Bass gives you bass so low that it putts out the lowest notes, so be careful. String Synth is a good solina/early poly synth string patch, so I can now check that off my list. Poly VI is the sound from Top Gun's "Take My Breath Away", and it's hard to get it NOT to sound like that. But, you can do it somewhat with a little tweaking.
This pedal really requires additional processing, in my opinion. It sounds glorious through a phase shifter, echo, chorus, etc. Make sure you have these to make the sound richer and more natural.
One final comment. I was slightly slower to pull the trigger on this pedal because I heard early feedback that it doesn't track well. Yes, it can sputter a bit on the patches with massive compression (that means you, OBX), and it certainly can miss notes on complex chords. But if you have ever played any of the early guitar synths or even the Osc synth in the GP-10, you will scream with laughter at that comment after you play through the Synth 9. It is a dream come true compared to almost everything else out there. Just remember, this comes first in your chain, PERIOD.
Conclusions: If you can hang with the sonic and versatility limitations of the ____ 9 series, this is clearly the latest and greatest of the series. It's fabulous. Just make sure you know what you want to use it for first and that it fits your needs.