After some reading, I did the following speaker upgrade for my 2015 spark EV:
JBL GTO429 (RMS 5W-50W or 35W, 3 ohms) from Amazon for $46.95
Pioneer TS-A4676R (30W nominal, 4 ohms) from sonicelectronix.com for $44.99
(for rear speakers): Meta 72-9300 from sonicelectronix.com for $5.99 x 2
Overall these speakers produce much-improved sound than that came from the stock ones. Because of their size, there is not really much bass that can be produced by these smaller speakers. I do think a subwoofer is needed for a fuller and more satisfying sound. Anyway, I am happy about the result for now.