Regarding the Tortuga discussed by bigshutterbug, you can try one of the Tortuga Audio passives using their
30 day audition policy.
Tortuga Audio also offers a preamp buffer that I auditioned through their equipment "tour" that is dicussed at their site on AudioCircle;http://www.audiocircle.com/index.php?board=85.0
In my system, the buffer added body and drive to my Goldpoint passive, without any significant sonic penalty. The Goldpoint is a backup unit I own that offers clarity but doesn't really measure up to my main
preamp, which is an SMc Audio unity gain buffered unit (like the SMc VRE-1), basically a passive preamp with a buffer circuit. Adding the Tortuga buffer to the Goldpoint allowed it to perform much closer to my SMc preamp. The buffer circuit improves the interface with the amplifier so that longer interconnects can be driven without penalty and that also IMO removes the drawbacks typically associated with passives, including loss of body and drive. Since their buffer is a stand-alone unit, you can always add it later, to whichever passive you decide to purchase.