"I'd also have to disagree with mgreen27; a passive is more likely to be transparent, so if the speakers are a problem (which I don't necessarily believe) that would not help."
I have no issue with you disagreeing with me, but for the OP's sake, I should probably go into a bit more detail. At one time or another, I've owned the entire Matrix series, including the 800, HTM and the Matrix HT package. They all used the same aluminum tweeter, and on every single speaker in the line, the same problem would occur. After a while, the highs just became unbearable. Changing the electronics made some difference, but usually not enough. I know some posters recommended tubes, but I wouldn't buy them in hopes of fixing any problems. Either way, the best results were gotten with solid state electronics. Rowland was the best, followed by Pass. In the end, the only way to get rid of the problem in the highs, is to process the signal heavily or match the speakers to components that are so dull sounding, it ruins the whole sound. When it comes to those old B&W speakers, there's 2 kinds of people. Those who listen to them, and those that can't. Its that simple.
With passives, the transparency argument looks good on paper, but in practice, its a different story. The typical passive customer usually buys one because they can't afford a really good active preamp, and they can't listen to a cheap one. 99% of the time, using a passive takes the edge off the highs. It reduces sibilance. You may be able to make an argument that a really high end passive, like a Placette, may have the transparency to be an issue, but its very expensive and not the norm. The Schitt passive is only $50, and not in that league. He already has a Pass amp, so if the combo doesn't work, very little money is invested. If you can't get the highs under control with a Pass amp and a passive line stage, you need to rethink your system or risk loosing a lot of money.
I know my comments may sound a bit direct or harsh, but in this case, I feel its justified. I know dozens of people who lost money trying to fix B&W speakers, myself included.
