During my time as an audio reviewer I've probably reviewed over thirty different preamps. Most were active, but a few were passive or switchable from active to passive. I'm still waiting for an active preamp that's undetectable in a bypass test.
First, I guess it would be nice to define what an active preamp is, then I'll explain bypass tests. An active preamp is a component that has circuitry that can amplify line level voltages over unity gain. Unity gain is the level of the signal when it enters the inputs of a preamp. Active preamps can use tubes, discrete transistors, or op-amps as their gain devices. Conversely, a passive preamplifier is one with no gain devices. Passive preamps can have transformers or other impedance-matching devices, and some even have provisions for remote controls, but a passive preamp can't amplify a line-level signal.
So what's a bypass test? A bypass test is when you insert a device in the signal chain so that you can A/B its effects in and out of the circuit. A preamp's tape loop serves as an excellent location to insert another preamp to see if the sound changes when the tape loop, which includes the preamp under test in its signal chain, is engaged. I've also done bypass tests where a preamp was inserted between a preamp and a power amp. Obviously volume levels need to be identical and any bypass test also involves an additional interconnect which can also have an effect on the overall sound.
Two "active" preamps that came very close to the ideal of "a straight wire with gain" were the Carver Lightstar and the Adcom GFP-750. Both could be easily switched from active to passive. And with both units the best sonic results came from using them in passive mode. The best feature of these preamps was that when you needed an active preamp, you could engage their active circuitry, but otherwise this part of the preamp could be bypassed.
Now, the main problem with passive preamps is that they can create impedance mismatches that can reduce dynamics and pace. Passive preamps put all the line-driving responsibilities squarely on the shoulders of the line-stage of your source device. Some DACs and Players lack adequate output capabilities to be able to shoulder the responsibilities of driving a long line without issues. Also sometimes you need a way to deliver more volume than unity gain, which a passive preamp can't do.
Purely passive preamps, such as my Reference Line Preeminence1B are ideal in computer audio set-ups where none of the components are more than a couple of feet away from each other. Short cable runs are also far less prone to impedance issues. In bypass tests I've been unable to hear the Reference Line in the signal chain. Unfortunately, all three of the preamps I've previously mentioned are no longer in production. Current passive preamps include the Transformer Volume Unit from Stevens & Billington who used to make the highly-regarded Music First passive preamp, Placette whose passive linestage goes for $1595, Creek who makes a $500 unit, and Channel Islands Audio makes two preamps with both passive and active circuitry. For DIY types you can find several passive designs that can be assembled for under $200.
Of course if your DAC has its own internal preamp, such as in the Wyred4Sound Dac-2 or Weiss DAC202 you don't need to have a separate analog preamp in your system - you can directly connect either of these DACs to your power amps, which is how I use them. Especially if your system uses exclusively digital sources, using a DAC with variable outputs not only saves you the cost of a separate preamp, but can also deliver superior sonics. Sonically, nothing is better than nothing at all...