Hey guys, seeing you're experimenting with the crystals, which is my area of interest for the last few years, I thought I would give you some of my knowledge/understanding/experience of how this thing work and how to achieve best benefits with them.
Actually, what brought my interest in this field is the guy under the brand name Audiophile Rocks, who is inventing crystal formulas for about a decade and is the nicest, most helpful and knowledgeable guy I've ever dealt with in audio industry (imagine cdacosta, but in the crystals field). I'm utilizing some of his creations with an extremely large benefit, especially the Terminos (RCA plugs filled with his unique formula) and we've been discussing the effects and principles for quite some time now (and no, we're not affiliated, I just like to give credit to whomever deserves it).
My current understanding of the working principle of crystals on our audio gear is in two domains:
1. Area/field effect - this is the most usual effect you're here experiencing, as it obviously seems/sounds that the proximity of crystals near the signal conductors alters the sound, based on frequency of the crystal. The most common type is clear crystal, which is tilting the tonality the higher-end of the frequency spectrum, cleaning/harmonizing it from any present pollution and thus perceivably enhancing it, but at the same time exaggerating it, so one has to be careful not to overdo it.
I think this is the same principle as some electronic field conditioners use, such as Nordost Q-Point Resonance Synchronizer and although this doesn't change the tonal balance as much, the mechanism of action seems similar, overwriting the surrounding EMI/RFI mix of frequencies to a single coherent one.
2. The other lesser known, but I would argue a potentially more important mechanism of action of a crystal is its physical/contact resonance characteristic. And here it gets more interesting, because in order to understand how is it actually working, I believe we ought to take a peek at quantum mechanics. In this realms, there's no such thing as solid matter, in other words, everything we perceive as solid, is just a sum of extremely fast oscillating particles (or waves, or both, right?
. Now, we have to take into consideration that each material has its own resonance characteristics and that is perceivably manifested in "sound" of the material. Just a simple knock on the material reveals its sound.
The reason why I'm writing all of this is because after countless experiments over the years in resonance management department, I've come to conclusion that the resonance of each component gets translated into the resulting sound. What is the mechanism behind it is beyond my imagination, especially in the digital part of the chain, but the fact remains that each time I change the resonance of any component of the chain, the sound changes with it. This applies to amp, DAC, streamer, network switches, routers (and their PSU's), power distributor, etc..
To go even further, I would differentiate between inherent and acquired resonance. Inherent is the material's own resonance frequency/characteristic and acquired depends on all layers of materials/bases, on which the weight of the component stands (usually those are some rubbery feet, then desk/rack, floor, etc, until the first decoupling/damping material, if there is one).
I'm of belief that to get the cleanest and purest sound, we should seek the "best possible" resonance management of our devices in the chain. For me that means to get rid of the negative/disturbing resonances - both inherent passive (metal cases of devices), active (actively vibrating parts, mainly in PSUs) and acquired (from materials on which the components stands).
Now I'm realizing that in order to continue this explanation of resonance management and how to choose the best approach for various devices, this post would have been several pages long, so I'll keep it short from here.
There is one elegant short-cut to get the resonance management to the right direction-> and this is to induce the "good" resonances. Crystals like quartz are strong natural resonators, which can transfer their natural/healing resonances and essentially override the inherent resonances of the material they're attached to. They act like a patch that can heal what's wrong with the material/component and bring more naturalness and calmness to the sound.
But in order to work really effectively, they should have the connected side absolutely flat and be firmly attached to the device using a very thin and neutral material. I saw some of you are using BluTack and although one can still clearly hear the crystal's contribution, you're leaving around 60-70% of its performance on the table. I strongly recommend against using the BluTack for this purpose (or maybe only for discovery reasons) and substitute it with some other, thinner and more neutral adhesive like some ultra/hyper thin tapes from 3M. The difference is night & day as you'll hear yourself.
Last thing - my preferred crystal type is smoky quartz, since this one doesn't tilt the tonality to the higher-frequencies to the same degree as clear ones (but still does) and perhaps cleans the surrounding EMI to a larger extent. Look for shapes like the Acoustic Revive QR-8 and beware of larger diameters, as they tend to color the sound just little too much.