New receiver or amp?

New here, and don’t know much about surround sound systems. I bought random speakers, and a receiver about 12 years ago, and wanted some opinions on how/if I should upgrade anything. I’m currently running:

Center: Polk CS1 (considering upgrading to CS2, or any other suggestions?)
Front: Polk T90e (monitor 60 equivalent I think)
Bookshelf: RTi4
Receiver: Harmon Kardon AVR 130
Sub: MTX Audio- SW2

Do I need a better receiver, or can I just add an amp? Any make/model recommendations? TIA

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