At
$900/pair, that's a fair price for LS50s. I've heard them and like them. Crutchfield's price puts them at a similar price to the Revel Concerto2 M16 speakers that shadyJ recommended. However, that price for the Revels was from 2016. What do they cost now?
Other small but highly recommended 2-way speakers in that general price range are the
Ascend Acoustics Sierra-1 (on sale at $678/pair!), and the
Salk SongSurround I at $895 (black or white) or $995 in your choice of 5 different veneers. Both these excellent makers sell only by Internet Direct. You can't find them in stores. Salk in particular does excellent work on its wooden veneer finishes, if that interests your mom. Both are highly regarded.
All the above speakers are easy to drive, and will do well combined with a sub woofer. I don't think you could do wrong with any of them.
FWIW, I have a pair of those Salks and when I heard the KEF LS50s, I was able to directly compare them. They aren't identical, but I liked both, and am happy to recommend either of them. I use mine as a 2-channel music only system in a large bedroom.
Those KEF LS50s will work well, as in rock out, with a sub.
They naturally roll-off below about 80 Hz. I don't think they require a high pass filter.