I've had many pre-amps both passive and active. I've always found that the circuitry in active pre-amps causes colorations, as stated above by Kevin from Upscale Audio. Sometimes this is a good thing, as in the case of the Wyetech Opal. Sometimes not. I would prefer transparency in my system to any colorations.
The active circuitry in active pre-amps is also subtractive in detail as well as transparency. I have to laugh at reviewers who say that the active pre-amp they are reviewing has "no sound at all". How would they know? They are only comparing it to the pre-amp already in their system. Reviewers who fairly review pre-amps have some type of DAC or source with a volume control that let's them test the credibility of the "no sound at all" statement. I have never seen any reviewer using this technique make that statement, and the conclusion they ultimately make is that they prefer the system without the active pre-amp.
Passives are more transparent, but inevitably limit dynamics and bass. I have found two solutions. The first is the RDP-1 from Z-Systems. It is a digital pre-amp that goes between your transport and DAC, and controls volume in the digital domain. It is also a digital equalizer. The second is the Silver Rock passive from the Swiss company Audio Consulting. It is a completely silver wired transformer/potentiometer with dual mono construction. Neither of these pre-amps limits dynamics or bass, and the transparency is stunning. TAS reviewed the Silver Rock a few months ago, and compared it to some very high-end active pre-amps. The actives were less transparent, and were grainy in the mids and highs when compared to the Silver Rock. They also commented positively on the Silver Rock when they reviewed the Halcro amps.
The Silver Rock sometimes causes grounding problems when put in your system, and is a bit finicky about other equipment. It also isn't the most beautiful piece of equipment around, has no remote control, and is very expensive for a passive. Although I would add that, judged solely on sound quality, it's a bargain.
Both are wonderful solutions to volume control in a CD only system. I have both at the moment, and will sell one in the near future after I decide which I like the best.
