Aqualung is only sold thru dealers. The few Aqualung items you see online are are being sold at their licensed dealers who also have an online presence.
My go to online is
www.scubatoys.com in Dallas or
www.leisurepro.com in NYC. The Aqualung sold at LP is cheaper since they're
not an authorized dealer so offer their own warranty instead. Everything else is mfr. warranteed.
Scubatoys is both an Atomic and Zeagle dealer so buying from them is like buying from a local dealer minus the sales tax - warranty coverage is the same. But really the prices are also except they usually have some really good deals on Atomic on Black Friday weekend so might be worth the wait. At one time Atomic was built in Huntington Beach so handy for service but there's no sales there afaik.
Both Atomic and Zeagle are now own by Huish Outdoors - they also own Bare and Liquivision. If you see a really good deal on Apeks regs, they're distributed in the U.S. by Aqualung so can be serviced worldwide also.
None of the 3 brands you mentioned ever have much of a sale - I can buy Zeagle from Leisurepro, Scubatoys or locally for the exact same price. I have heard Zeagle allows 10% thru local dealers but mine acts like he's never heard of it. So I buy from Scubatoys...
One of the larger online retailers is in Irvine - Scuba.com. Have never been there and there's some inconsistent reviews here - mostly on the mail order side. I ordered some boots from them once pre-trip and canceled two months later when they were still sending backorder notices. Probably be OK if you picked up your stuff there.
Know about divebums.com? - it's almost all things San Diego diving - except sales of gear.