Well @RashDecisionAudio, first things first.
I am assuming you’re getting a quack, that’s why I asked to confirm the frequency, because 700K is way past the audible spectrum. I checked the article you mentioned and it looks like TC Electronics set the centre-frequency at 700 Hz plus or minus a few octaves. That is an average value for most standard-tuned, steel string guitars. (For nylon strings, Nashville tuned, baritone guitars and upright basses those values could be a lot different. Hexaphonic systems will have one piezo per string, so in theory you can have 6 trouble frequencies.)
So, the first thing you need to do is determine whether or not you’re having a quack and determine the frequency (you may employ a computer-based spectrum analyser for that.)
The best way to deal with the piezo quack is to not let it exist in the first place. Not being overly exhaustive here, one must be aware that the quack – a slight distortion that usually has a centre-frequency that can be anywhere from 250Hz to 6Khz – has 2 main causes: mechanical and electronic (impedance mismatch).
The first has to do with how the element and instrument saddle with it are built. Piezos can be made of titanium, zyrconium – Gibson even attempted to use Kynar film for its piezo elements. Each has different properties, and the way it is inserted in its casing plus the way that casing is installed in the instrument make ALL the difference in the world. If the guitar bridge cavity is uneven and/or the bone above it is not straight, you’ll get a quack that will make you think Donald Duck is Sade Adu. There are but a couple of piezo undersaddles in the market to my knowledge that are built in a way that there’s no mechanical vibration inside. The downside of them is that, if they are hit hard, they almost instantly break beyond repair, so a lot of people think they are actually bad…
Then, even with the best piezo sensor in your guitar, you also need to consider that its very high impedance must be matched at the preamp/amp side. In most acoustic amps, it’s the Hi-Z input. The Mod has an input impedance of 1MΩ, so it should match most piezo systems (bass included.)
(I sifted through a lot of posts on Acoustic Guitar and other forums, and only ONE person mentioned impedance as a cause and just 2 or 3 talk about mechanical factors. Then, apart for the deluge of hardware solutions, some people suggest employing materials – including gel!! – to have the piezo saddle sit uniformly in the instrument. If what energises the piezo is mechanical vibration, why in the bloody hell would someone smooth that out? That kills quack for sure, and probably about 70% of the piezo efficiency with it.)
Lastly, consider this: quack emerges due to the causes above and becomes more prominent if sensibility and gain are not optimal, which makes the player hit the strings harder. Piezos respond to mechanical pressure, but their downside is that the energy (voltage) rises too fast, so the ‘attack’ tends to be harsh, too fast, and sound unnatural. Whereas many pedals (such as Fishman Aura) and DIs (LR Baggs Para DI is great) can provide some relief, I only know of one person who address the actual physical characteristics of piezo elements for audio applications. Rolf Spuler’s WooDI does precisely that: it delays the initial attack and spread it over time. The result is subtle but incredibly musical. (Disclaimer: Rolf was my personal friend and we discussed this piezo spread technique extensively before his passing in 2014.)
Those two things help, even if they do not “cure” the problem. A simple boost of bass and trebles coupled with cutting off the resonance (quack) frequency should do what you’re looking for.
Even if a plugin specifically called “Notch Filter” doesn’t already exist for the Mod, there are several compression and EQ options that can do just that.
To fix that absence, one of Mod’s strengths is that it built over open-source standards and therefore allows anyone to build a plugin of their liking port it to Mod. Considering that a notch filter is essentially and solely a parametric filter – and also that there are many Parametric EQs whose code is open, it is relatively simple for someone to just employ the code for the parametric filter and “box” in a single plugin. Instructions and links to API documentation are available here.
Furthermore, MAX users can also create their own effects and port them to Mod – which is what I will do, since my half-baked brain cannot do programming. Instructions can be found here. More about Max’s gen~ object here.
Finally, bear in mind that in many cases the notch filter is at the amp, since it also caters to microphone resonance. They are mostly fixed-Q filters. Even Jon Gomm’s beloved and much worshipped Trace Elliot TA-200 amp has one at the preamp stage, and you can’t change the Q factor.