I do not think that it makes a sense to searched for “LF for use with Vitavox S2”. Do you think that there are good LF to use with Vitavox S2 and there are good LF to use with other drivers? I do not think so. The good LFs are good by themselves and they might not be good for any practical driver.
Whatever solution you might chose be advised that it is hardly unlikely that a single channel would be able to handle everything from 500Hz and down. To produce 40Hz (real 40Hz) and 300Hz it require quite different capacities and quite different strength from the drivers and from the enclosures.
"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche