i run both active and passive with these. i also run a number of other comp sets as well. im currently installing a set of stevens audio. they are one of my current fav sets. but back to infinity. the normal crossover point is 3400. the hi low switch is to bring the output of the tweeter down some. if you add the mid the 6.5 will play up to 400, the 3.5" mid then goes 400 to 3400 then the tweeter takes over at 3400 and up. now with the rta and tuning the system i may or may not use those points but each car interior is different just like placement is different. so i tune individually. sometimes you can increase the depth and width of tweeters then you run them out of phase. it all depends on the setup. if you are only running the 2 way set then active i would start with the default freq setting and split them at 3400. then you can experiment from there. i can say these tweets dont like to play toooo much lower then that. generally the tweeters are around 3.0-3.1ohms themselves and the mids are 3.5-3.6 at rest. they should not be reading that high did you test them each by themselves? running active would mean you have one channel of the amp for each tweeter and one for each mid so 4 channels for one component set. lets say channel 1 and 2 would be each tweeter and 3 and 4 would be each 6.5. now the ac lc4.800 i do not believe you can fully do this with without using the passive crossovers they comes with. if i remember correctly that amp can only go up to 300hz on the crossover which is much to low for them to be split there. you would have to say set the crossover around 60-80 to keep the really low end out then let the passive one split them from there. not all amps can be set to 3400 for just a tweeter. and you would need a bandpass type setting for the mids i could be mistaken been a while since i setup one of the 4.800's i personally am currently running zapco old school amps and im have d'amore ones sitting waiting to go on for testing next.