Passive preamps can sound amazing, but it takes much more care in component matching. You can in essence relying on the output of your source (in my case, my DAC) to drive the amp (in my case Coincident Frankstein 300b). If the input sensitivity of the amp is high enough and the output of your source is high enough, then a passive preamp can sound absolutely glorious (keep in mind that you need to select cables very carefully as well and avoid any longer lengths). Get it wrong and it's going to sound flat and uninspiring.
On the other hand, a really good active preamp will remove most, if not all, of these constrictions and can very well make your entire system sound better.