Some information on setting up speakers for my PC / General use

Never wanting to invest too much in Thailand, a couple of years ago I bought a little sound unit for my PC. Basically a blue tooth Subwoofer / amp and 2 x passive speakers. Sherman brand (SB-55T ....50w RMS) I think it was under 2000 baht, but sounded surprisingly good, powerful and has been reliable. The sub woofer is good but the 2 x passive box speakers a little thin. Just wondering what sort of improvement I would get from getting rid of the original speakers and hooking up a couple of better quality bookshelf / desktop speakers. I was always under the impression the amp itself doesn't really vary much, its the quality of the speakers that makes your sound. That could be completely incorrect. Not after audiophile quality here, just a richer sound. I don't intend to spend a lot.....can pick up a couple of decent secondhand small (30-60w) Denon's or even JBL's for under 2000 baht.

There are no specs on the amp or speakers....I see a lot of the passive bookshelf speakers are 6 ohm, some 8 ohm. Is that going to matter greatly?

Any advice appreciated