What frequency setting for sub woofer

My speakers have a frequency response of 28 - 25Khz. I also have a sub woofer and Im wondering what frequency that should be set to to get the best out of it please.

Best to do this by ear, I think, but I would use a frequency just a little higher than the quoted lower figure for your main speakers as a starting point and take it from there.
With a lower figure of 28Hz I presume your main speakers are a reasonable size, but at a wild guess you could perhaps try the sub at around 35, and adjust it up or down from there.

Even though I use a BK XXLS400 sub I found some nice tips on the SVS website.

No they are quite small floorstanders, PMC twenty5, 23i’s. The bass is superb for the size