If it has a gain adjustment on the amp, that also be nice.

If you're using passive crossovers, then you'll need a larger amp than if
you used active crossovers.

For instance, if you want to provide 20 watts to your tweeters, then you
only need to buy a 20 watt amp if the high-pass crossovers are built into
the amp or head unit (active).  If you're using passive crossovers, you'll
need *roughly* a 75-100 watt amp (depending on the xover point) to provide
the very same 20 watts to your tweeters.

The reason for this is that the amplifier still has to amplify the entire
signal if you're using passives.  With actives, the signal is already
filtered before it reaches the amplifier input.

So for this reason, and other reasons too, I strongly suggest using active
crossovers when using a dedicated tweeter amp.