I don't think your speakers are really the issue. Sound quality is very subjective so I tend to rely on measurements a lot more than people's perceptions.
In my opinion, your first step is to invest in a Umik-1 mic and learn how to use REW that is freeware.
Acoustic Measurement Tools: UMIK-1
REW - Room EQ Wizard Room Acoustics Software
If we can see some measurements, we can tell a lot more about what the issue is, and what might be the solutions.
Without taking measurements, if I were to take an educated guess, one of you best bet is to purchase the Denon AVR-X3800H, that is currently on sale in North America so it may be too in Australia, and then purchase the Dirac Live licenses. Then you can customize the target curve to suit your taste, in terms of "mellowness".
Crossover design is not rocket science, but it is a lot more than the average hobbyist to tackle, especially when trying to retrofit it to speakers like yours. So, I would suggest you the route of Dirac Live, another low cost AVR that you can consider is the Onkyo RZ30, that has two discrete subouts, but it can only do 9 channels of processing, the Denon can do 13.