difference between an active and a passive preamp?

I think that the output impedance is only half of the equation here. The other half is how the waveforms are generated from the source. Can you tell us the make/model of your DAC? Also, what kind of amplifier? I general don’t like to recommend passive preamps unless the source device has a very good Class A output stage or tube output stage. Generally, sources that use op-amps only as output stages may still have somewhat of a digital sound. It really depends what’s on the output stage. Sometimes there are excellent op amps and circuits (i.e. AD797 or something similar). The OPA2134, which is very popular, is okay but has more of a laid back sound and doesn’t have the resolution or separation-of-instruments as other circuits available.

My DAC is Musatex Bitstream. Amp is Class D. Thanks. 

Interesting.  If you have the "Data II" update, it looks like you have a Class A circuit.  The Musatex probably uses FET devices (the MELIOR BS-1 used FET).  A passive preamp may work well here.  If you're not sure, you can check out the Schiit Freya.  With that, you have your choice of passive, JFET or tube gain stages.  It's probably in the budget that you're looking for.  15 day return policy if it doesn't work out.

check out the Schiit Freya. With that, you have your choice of passive, JFET or tube gain stages.

+1  auxinput

Cheers George

Thanks! No, mine is a stock unit.

I am leaning towards the Tortuga Audio but have not made my mind yet.