I'm a little confused. You have a powered mixer and an amp? Are you bypassing the the amp section of your mixer?
The G5 is underpowering those speakers by about 17%. Not a big deal, but you will gain cleaner bass and more detail in the highs and mids if you were to upgrade the amp to something pushing the full 600 watts/ch @ 8 ohms that your speakers are designed for.
I run EV SX300s as my mains as well, and power them with a QSC PLX1804. You would notice the upgrade if you could sell the G5 (retail $400) and upgrade the PLX1804 (retail $450). If you got lucky, you might even break even on the sale and rebuy, especially of a used amp.
Of course, the JBL 615 would be an upgrade in low end. You are talking a 12" woofer that's good to about 50Hz versus a 15" woofer that's good down to about 40 Hz. Question is if that is worth the extra cash
As stated, you would clean up that low end a bit with the amp upgrade keeping the current speakers. Just don't think you are ever going to get thunderous bass out of any 12" speaker. You are looking to tune it to more of a punchy bass sound with a long throw. My sub quit on me about 3 weeks ago, and I've been getting by fine in a 6000 SF room with just the EV SX300s.
The mids and highs of the SX300 and JBL are going to be almost identical.
Finally, as stated, not sure what you are doing with the powered mixing amp. You could sell that, buy a new mixer without an amp, and still have money leftover to start saving for a sub. A single sub with the sx300s properly powered by a 600 watt amp is exactly what I run, and it sounds sweeeeet!