Question regarding hooking a sub to active speakers with no sub input

Hey lads!

I’ve been meaning to add a subwoofer to my pc set up, currently consisting of a pair of headphones, a d30, a jds labs atom and a pair of swan m200mk3’s. The issue is that the swans dont have a sub out, and im not currently in a position to get a proper preamp/reciever etc with enough outputs for the speakers + sub.

One solution that I’ve read about is simply connecting them in a line a’la:

DAC/Pre-amp (Should work with the atom, right?) -> subwoofer input -> subwoofer output to speaker input.

The confusion i have with this is pretty much regarding the input/outputs of the sub that ive heard need to be ‘speaker level’ inputs, and also, if the crossover functions properly with such a set-up? - does it filter out the necessary frequencies for the sub and only sends the higher ones through the output to the speakers? or would it only remove the higher frequencies for itself and still send the full signal to the speakers?

Say if i was to buy a Sunfire sds8 or svs sb1000, would it work to:
Atom rca -> sunfire rca inputs -> sunfire rca outputs -> m2000 inputs?
and would the crossover function this way?

Any suggestions on how i can set it up and, if these subs inputs are not speaker level, any recommendations?

Thanks a lot in advance
//Audionewbie