I've been agonising on how to give you an informative response, since I'm not sure on your depth of knowledge pertaing to filter network theorems and associated mathematics.
I will start with a few basic facts hopefully without getting too heavy.
Filter networks usually come in the form of T or TT (pi). The quasi parallel sections will tend to want to "resonate" and form peaks (Q) and troughs (nulls) before the attenuation commences, -IF, of course, that is what the application requires, otherwise, the incorrect component values have been chosen. If the application is audio, then that peak/trough should not be a desired outcome. It is actually distortion, and it is audible. Some manufacturers use this trick on cheap subwoofers (they have a wild and woolly sound when played on their own).
Consider this; if you are attenuating, how can you have more out that what you are putting in?
These types of issues forced me to formulate calculators that allow me to instantly see the changes in curves for each component swap. This has made my crossover design process so much easier and quicker with not many corrections. I can also tailor-make stepped slopes if required.
The other bits of info I can offer are:-
* With large amplitude signals, metalically cored inductors will saturate and cause audible distortion. Try large air-cored units instead (place them outside and make them a feature).
* Ruler flat frequency responses will never be achieved with passive crossovers. Only digital manipulation can achieve this.
* If someone thinks they can hear variations less than 2dB, they are deluding themselves. Yes, it's a harsh reality, and it gets worse with age. Consider that the tolerances of the components will give you that alone.
Some of this information you may like, and some you will not. But they are the results of my experiments and experiences, and will stick by them. These days I realise that the time I have left is precious and prefer to enjoy listening to the music ( yes, yet another cliche') on a nice system rather than the system. Also, having just returned from a developing country, one sees things that put our first-world lives into perspective. I think most readers will understand what I mean. So yes, spend on what makes you happy, but always know your limitations.