difference between an active and a passive preamp?

Passive has no gain stage.
Active has a gain stage (at least one)
In all other ways they are about the same physically.
electrically though, they are different.
The passive preamp depends entirely on the source component to 'drive' the amp.
So using a passive preamp means you MUST have a robust source device. (or the result will be thin, weak sound)
Generally most CD players can drive a passive preamp 'good enough' That is it will sound no worse taht an active preamp.
For a passive preamp to excel.. the source must have a great ability to drive the amp.

With an active preamp none of that matters. any source can be used and a good result will occur.

Some sources actually state i nthe specs do not use with a passive preamp. (nice to have that info. My Audio Research PH-2 specifically states do not use with passive pre._
So.. IF you can find out the ability of your source to drive the amp, a passive will be good..
If it cannot do it well, then you either have to get a source that can drive a passive, or get an active preamp.
When you get up there in money spent on a preamp.. the actives get pretty good.
In the lower ranges is where the passive shines in comparison to an Active pre. IMO,

King Rex preamp under $400. As transparent as a passive with tube like mids and deep bass of the solid state pre it is. I believe it has 15 db of gain. A real fine sounding unit the sounds well above its price.

There are a lot of caveats when using a passive preamp. As Elizabeth said, you have to make sure your source component has a very robust analogue output section with low output impedance. In addition, your interconnect lengths from source to preamp to amp (short as possible) and capacitance of the interconnects (lower the better) and the input impedance of the amplifier (higher the better) are critical as well. There are passive preamps made of pots, stepped resistors, LDRs, transformers, and autoformers. I prefer the latter three. If all of these are taken into consideration and executed correctly, a passive preamp is better sounding than any active preamp I have ever heard. But I have not heard the mega expensive $15k+ preamps like darTZeel, Atma-Sphere MP-1 with upgrades, etc.

Very nice clear responses...the way they build upon one another...nice to see someone get a direct answer(s) to their questions. Educational too....One for the archives!

The input impedance of your amp is 20k ohms. IMO this is not a great match for a passive preamp. There could be exceptions, but I am not aware of any.

On the other hand you could use a buffered preamp. This uses active circuitry with no gain. The active circuitry greatly minimizes the risk of impedance mismatches. There are tube buffers and solid state buffers. A few solid state options that come to mind are the Pass B-1, Burson Audio, and Horn Shoppe Truth. The latter of which I own and can state that it is extremely fast, detailed, and transparent. It gets out of the way which is what I believe a preamp should do. However, I would not classify it, as well as other solid state buffers, as having a tube like sound.

Also note, not all buffers have a volume control. If you desire something "tubey" you should probably stick to an active tube preamp or a tube buffer with volume control.