If they're not mounted in a corner, move them. I have a pair of 8" two-way speakers with sealed enclosures that are powered by an old stereo receiver from 1978- while I don't sit there and think "The separation is wonderful/terrible", it sounds pretty good. For a garage system. Best one in the neighborhood. That I can hear from the front yard that's close to 100' away.
Spending two grand on a receiver isn't going to get you where you want to be- that's up to the speakers. Outdoor speakers are made to provide sound. Not great sound, sound. If you want a lot of sound (SPL and/or frequency range),
This line from the Amazon ad is part of the problem- "Combine this with minimal wall projection..."- if you wanted great bass, you should have bought a system that comes with a sub or produces more low end. One way to produce more low end is by reflecting from the walls- since bass isn't very directional, using the wall(s) nearby is an old trick to make them sound better/bigger. Using one wall helps a bit, using a corner where two walls meet is better and if you can place them where two walls meet the ceiling, you add even more- that's how I'm getting the low end in my garage and it's not finished, just has insulated rafter spaces. It does have a lot of things on the walls that disperse the sound because, after all, it's a garage and I have woodworking tools and machinery.
Don't be afraid to use the tone controls or equalization but do it right- make sure that you don't boost signal the speakers can't handle.
