subwoofer advice

Snapsnap, I think you are going about it the wrong way. What you should do is get another pair of passive subs, a subwoofer crossover like the one JL Audio makes and two commercial stereo amps like QSC PLX 3602's. These amps drive subs way better than any plate amp and they are very reasonably priced. 
You could also get a digital preamp that provides subwoofer crossovers like the DEQX Premate. This is better than any analog crossover and it gives you room control.
 Forget trying to match powered subs with passive subs. It will not work well. You either go all passive or all powered. I personally prefer passive. It gives you a lot more flexibility and the opportunity to make your own subs by either Kits or your own design at way lower prices. 
If you have any questions about doing this feel free to message me.

Mike

Snapsnap, if I understand correctly, you want to power your passive subs with the amplifiers which are already in your active subs, is that right?  

If so, that's extremely unlikely to be feasible.  The amps in your active subs are matched to the impedance of their woofer, and adding a second woofer in parallel will probably cause your amp to shut down.  And adding that second woofer in series will probably cut your amplifier's power in half.   

I suggest powering your two passive subs with an amplifier dedicated just to them.     

Duke

Thank you all for the advice - seems like I'll have to go either passive or active rather than a combination. 
I will have to do some research on what amp would be suitable for subwoofer duty, and how many are required to drive multiple passive subs. On that point, will the additional amp(s) be connected to the main amp or to the preamp? 

snapsnap, I use two QSC PLX 3602 amps to drive 4 subwoofers. They put out 1800 watts/ch into two ohms. Each channel has a gain control and there are balanced inputs. They are AB amps with switching power supplies. They are $1500 each. They make smaller versions of the same amp for less money.  The only possible problem is that they do have internal cooling fans. Mine are on a shelf under the floor in the basement so I do not hear them at all but they might bother some people. Theoretically, driving 4 or 8 ohm subwoofers you probably do not need the fans. You could disconnect them keeping an eye on the temp. If the amps continue to run cool than the fans are not necessary. You can always hook them back up.
Dayton also makes a line of Class D plate amps. One even has DSP.
I do not like installing electronics in Subs and I prefer AB amps with very low output impedance (high damping) 

Snapsnap wrote:  "I will have to do some research on what amp would be suitable for subwoofer duty, and how many are required to drive multiple passive subs."   

I use the Dayton Audio SA-1000 for the Swarm.   Assuming your passive subs are 4 ohms each, the SA-1000 would put out almost 500 watts into the 8 ohm load of your two subs in series.  If your subs are 8 ohms each, you can connect them in parallel for a 4 ohm load, into which the SA-1000 puts out a little less than a kilowatt.  

Snapsnap:  "On that point, will the additional amp(s) be connected to the main amp or to the preamp?"   

It depends on what kind of inputs the subwoofer amp has.  You can find subwoofer amps with either and with both.   

Duke