What is wrong with the Polks you already have?
You said you have three speakers two polk S10 towers that are at least 15 years old, and a polk CS275 (age unknown).
Is there anything wrong with the S10s?
If not, is your receiver a 5.1 or 7.1?
If nothing is wrong with the S10s, just buy two or four small speakers for surrounds (plus wall mounts or stands and cable).
I've got new speakers (relatively speaking - less than 5 years old); Mirage Omni 7s (front R&L), Mirage Omni Center.
I've got old speakers; dBx Soundfield Vs from the mid 80s.
I've got very old speakers; Criterion L8s from the early 70s.
All still work nicely, sound good (even at high SPL), do not appear to have any deterioration. So if it is just their age!