How do you judge what is a good one and a "not so good" one?

So many brands out there, how to choose?

Pls share.

IMO, there are few duds out there, and it's usually reflected in price. of the decent ones, you won't hear any audible difference between them. so it boils down to choosing one from a reputable brand, and one that offers the features you want, at the price you want.

of the budget end, brands like coustic and more common namebrands (eg: pioneer, clarion, etc; many of which may be discontinued now, so only available secondhand) are fine. then you move up to more specialist brands like phoenix gold, mtx, audio control, etc.

but not many brands offer them these days, so consider secondhand.

you can view their specs, and probably most important from an SQ perspective is SNR (signal to noise ratio), which, at >70dB will be perfectly fine.

Thank you Shiny for your information.

My own thought has been confirmed according what you say.

does it going to make a lot of difference compare to passive xover in term of sound quality?

If you want a good, cheap XO get a Sony XEC-1000, they are a good unit and you should be able to get one for less than $100.