I'm not a techy but understand that passive's are essentially just volume controls which in theory, add or subtract little to the signal path between source and power amp. As to whether they are any good, I think you need to judge this one for yourself.
I used a Creek passive pre amp (OBH22) for a number of years between a squeezebox touch and a Roksan power amp. I then replaced it last month with a Linn all in one streamer. The biggest criticism of passives is that they can sound a little sterile or clinical and I tend to agree somewhat with this suggestion.
You may, however, find a different combination of passive pre with power amp gives very different results - it's one of those 'suck it and see' answers I'm afraid.
Personally I’ve always found passive preamps a little flat sounding, that said I’ve not heard and highly rated ones.
What amp do you have? For a similar price you could get a Croft 25 Basic, light on features but in my opinion unrivalled for sound at the price. An active valve pre just gives that bit of life I like, I have the model up paired with a Quad 909 power amp and love the combination.
What are you actually looking to achieve? Improve the sound? Any areas you feel are lacking? Or is it for features? What are you using as a pre now?
The success of passive pre-amps depends largely on the source that you have and to a lesser extent the power amp that you have.
When I tried a Prometheus TVC passive amp with a Musical Fidelity phono amp - the one in the tube casing - it sounded really bad. All the dynamics and life in the music were totally robbed. Swapping to an active pre-amp made the system sound much better. When we used a Lyngdorf CD player in the same system with the TVC it worked well and dynamics were fine.
When I tried a Lightspeed LDR passive in my system I prefered it to the TVC due to the LDR being much better in the bass and treble - better clarity, less mush. The strongpoint of the TVC was in the midrange where it sounded pleasantly transparent. It's possible that my sources were overloading the TVC and saturating the bass. My sources have a very high output due to them being professional gear.
With CD players and professional sources - sources with relatively high outputs - resistor based passive pre-amps are great. Dynamics are fine. Clarity, transparency are great and the prices are laughably low. With phono amps like the Musical Fidelity - sources with relatively low outputs - avoid passives at all costs and get an active pre-amp.
I've read that amps with high input impedances 50k ohms to 100k ohms suit passive pre-amps more than amps with low input impedances 10k ohms to 20k ohms. My valve amps have 100k ohm input impedance and my solid state amps 20k ohm. But both sound fine with passive pre-amps in my system. I've also read that passives are best with short interconnects.
Passive pre-amps in the right system are better than active pre-amps. Active pre-amps in the right system are better than passives.