How smooth does it need to be?
One of the big issues in crossover design is the attempt to provide a very smooth, flat frequency response... which is, for all intents and purposes, not possible with currently available drivers.
But how much variation can there be before it starts sounding like crap?
First we have to think about how sensitive our hearing is. Most people don't detect volume changes well at all...
If I used a signal generator to produce a steady 5oohz tone then bumped it up and down by 1 db in loudness, it is extremely unlikely that anyone would notice. At 3db about half the listeners would hear a tiny change, the rest would be uncertain. At 6db most people would hear the decrease but not the increase. At 10 db most people would hear the change very clearly.
Using two signal generators with unrelated frequencies, say 500 and 800 hz, changing between them almost nobody would notice a 3db difference. At 6db a small number of people might notice, at 10db about half the listeners would notice.
So the truth is that our ears really aren't that good at detecting changes in volume... In fact, most people can't tell the difference between a frequency response of +-8db on the usual 20 to 20 khz scale and a perfectly flat response.
Then we add in the complexities of music, where (until recently) there were large variations in the sound levels of the instruments, singers etc. and any errors in frequency response are masked even further. Lately, with the loudness wars, with everything the same volume, it's a crap shoot with most people still not noticing +-6db variations.
Finally, add in the room itself. A room, no matter how well designed, is a real minefield for speakers with reflections, standing waves, cancellations etc. So even the biggest variations in frequency response are easily masked by the acoustics.
So what do we think is a reasonable goal for frequency response design in a crossover/loudspeaker?
I'd say that any response curve within +-6 db should count as acceptable...