Ian Bell wrote:

Very difficult to design a filter that changes amplitude with frequency
without affecting phase. Laws of physics type stuff.


Easy enough to do with a digital signal.

A quick demo would be to low-pass a file with Goldwave or similar,
reverse the file, filter it again and reverse it again.

In the analogue world:

You can keep the low-pass and high-pass outputs in phase
with a 4th-order filter. I got a cheap Soundtech active crossover,
which does this, on ebay. It's easy enough to build one though.

There's some interesting stuff at Siegfried Linkwitz's site:
http://www.linkwitzlab.com

--
Roger.