UltraSonic Damping Instruments affect a system's sonic signature mostly by helping to reduce the system's sonic signature and bring out a more faithful rendering of the musical event.
UltraSonics are the current generation in a linear progression of damping instruments of which each design has improved upon the previous. Some improvements have been sonic, some functional, some a combination of both. UltraSonic Damping Instruments have achieved a degree of perfection that overcomes any of the particular drawbacks of earlier designs.
The "flavor" you refer to is more along the lines of subtle differences in dynamic and microdynamic resolution and impact; all are quite linear with frequency response. The descriptions on our website are straightforward; you don't really need to try to read anything into them.
7308s are typically extremely sensitive to micro-vibrations; likewise, they are extremely sensitive to microdynamic nuance and subtlety in the music that is often somewhat masked by micro-vibrations. I recommend Preamp UltraSonics or UltraSonic Rx to bring out the very best in these tubes, though any of Herbie's damping instruments do very well with virtually all 6922-type tubes including 7308.
Differences between the various damping instruments are difficult to explain and differences will vary between different tubes, components and systems. Here is a brief overview, though:
Teflon HAL-O: Herbie's first product that introduced Herbie's Audio Lab to the audiophile world was for several years our reference product. HAL-Os have no particular sonic drawback with small-signal tubes and are overall very linear and do a great job of reducing microphonic distortion. They're fast dynamically, but compared to lickety-split UltraSonics, can seem dull in comparison. The major drawback with Teflon HAL-Os is their use will larger and hotter-running tubes--they don't maintain as well of a grip under extreme heat and the silicone isolation pads sometimes don't hold up over the years with extremely hot-running rectifier and power tubes.
MidFi+ Damping Instruments do have a slight "flavor" to them, tending to induce a subtle, gently-rising deep bottom end. It's not bloopy or anything and seems very linear. Though named "MidFi," they're really quite hi-fi. For tubes that aren't overly microphonic, they're a great bargain and provide a listening experience overall just about as enjoyable as any.
PEEK Damping Instruments tend to emphasize dynamic impact in music that is dynamic. With subtle music, the subtleties come through. PEEKs have more of a unique characteristic of being more sensitive to placement on the tube, and thus lend themselves somewhat to "tuning" by adjusting their placement. They are sometimes a little "slow" with microdynamics compared to UltraSonics, and the material cost makes them somewhat cost-prohibitive compared to UltraSonics. They are presently sold basically "at cost" to recover material investment.
BlackBery HAL-Os have no particular advantage over Preamp UltraSonics, which will ultimately replace BlackBerys in the product lineup. Sonically, BlackBerys are sort of picky about the component they're used in--they do beautifully, almost magically, in some preamps, CD players, Hammond organs, tube microphones, and other components. In others, they do relatively poorly.
Standard UltraSonics are our basic default recommendation for all vacuum tubes. They're very neutral sonically, very revealing of the source.
For small-signal tubes, Herbie's new Preamp UltraSonics tend to deliver a sonic presentation equivalent to standard UltraSonics, with a little more capacity to overcome microphonics. In some systems, they'll give a little less harsh of a result than standard UltraSonics.
Guitar Amp UltraSonics are designed to overcome vibrational torture chambers like combo guitar amps. They do the job very well, yet perform amazingly well in home audio systems, also. Every bit as engaging and musical as standard UltraSonics, they sometimes tend to provide a slightly more sense of "air" to the music.
UltraSonic Rx is overall our best damping instrument. If they were not more expensive than standard UltraSonics, Rx would replace standard UltraSonics in our lineup. Claiming they are our "best" is based on personal listening experiences. Several customers have auditioned Rx and standard UltraSonics, and in every case the Rx version prevailed. Differences between them are quite subtle though, and sometimes not readily apparent with quick A/B comparison. A more readily apparent difference occurs with systems that tend to be overly "dry." Not necessarily "analytical" but artificial-sounding as opposed to being organic and fluid. Rx Damping Instruments tend to cure this, thus the name, "Rx."
Preamp UltraSonics, Herbie's newest product, is an attempt to achieve the musicality of Rx with small-signal tubes at a more affordable price. I believe we have accomplished that goal, though Rx still provides just a tad more of a "there" there that I can't quite put my finger on or articulate just with words.
SteveHerbie's Audio Lab