Cnut, the "flat" I was referring to is equal output from the speaker system over the entire frequency range with a given constant input.
Raw drivers will output more sound at some frequencies than at others. In many high quality speaker systems, the manufacturer has designed the passive networks to compensate for these non-linearities.
Now, there may be little of this in your speakers. Your internal networks may be nothing more than simple frequency separation and in that case, the active xover will give you far more control over the resulting sound and no degradation.
Cnut, you are correct that room dynamics can significantly impact frequency performance. Those controls for tweeters are there to compensate for rooms either full of sound absorbing materials or with many hard reflective surfaces. Active operation, however, will do a better job that those controls. You simply get much more control.
Since the manufacturer of your speakers suggests going active, that-s probably the way to go. My comments were really about folks who were contemplating going active on speaker systems where the manufacturer does NOT recommend active operation.
Lastly, I-d suggest the simple way to solve your problem is to purchase the two way monitors and then get a high quality powered sub. The high quality powered sub, by the way is active! Your active xover will supply the line level signal for the sub and then it will supply the line level for the mid-range and tweeters.
Cnut, if you go this way you only need a two way active xover and two stereo amps. Life just becomes so much easier/simplier.
Here is an example of a two way active xover:
http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=245-862
There are more expensive units to be sure, but many folks have reported successful results with these.
Hope this helps -
Jerry