I got two pairs from MusicDirect for $200.00. I tried them first in my B&W 803N. I plugged them in the positive terminals of both low and high frequencies. The difference was noticable. The bass got tighter, the sound stage got a little bit bigger and the high frequencies got cleaner and more defined. These speakers are driven by a Threshold S550e which has the Stasis system and low feedback. Since these devices are designed to eliminate the back energy wave with low feedback their effect is not as impressive.
Then I moved them to my B&W 801 Matrix S3 which are driven by a pair of Sonic Froniters Power 3 monoblocks. Placed in this system their effect was very distinctive and good. Basically I had the same effect that in my other system but in a lot higher degree. They made the system sound sharper but without lossing the sweetness of the tubes.
I think thery are a very good tweek specially in tube based systems.
