I have been using several duffels from Eagle Creek, Gregory, and Patagonia (below). All have a low-pro appearance, are available in ~30L - 100L sizes, and are substantially less than $500. So far, they are very durable in use including airline travel, equipment hauling, and careless-kid use. Many include integrated, stowable, or removable backpack straps. My criteria includes lightweight, and preferably compressible.
What are you planning to do with it? Weekend travel? I see you listed Saddleback Leather in your list of options. Look at eBay for some great deals on their bags. I picked up a Large Briefcase on there for about $400 (United Airlines actually paid for it, but that’s another story) for travel to/from North Dakota for work and it holds more than I’d like to carry on one shoulder. I can say without a doubt that you won’t be able to hurt any of their bags unless you intentionally light it on fire and send it on its merry way to Hades, and even then I’m doubtful that it couldn’t be repaired. I’ve looked into their duffel quite a few times but haven’t been able to figure out a way justify it to the boss (i.e. wife :big_boss:). Seriously, I think Dave and his crew work really hard to put out quality products that will outlast the lifetime of the person that buys them, and they seem to stand by that with their 100 year warranty.
My main luggage is a Mountain hardwear Juggernaut 85. Fairly heavy duty with wheels. Mine is all black, so a little tactical looking. Not sure that is your goal, but it could be a great plate carrier, AK and assorted item bag. It handles 50-75 pounds readily.
Everything Patagonia I have used is solid. I have 15 year old parkas that I still use and some shirts, stained and worn, that I cannot give up. Their BlackHole bag series is the ultimate huge bombproof duffel, in a variety of sizes. They can be found in a series of different color templates and sizes. One of my favorite bags is a now discontinued dry pack in a smaller backpack format.
I have a 15 year old Uncle Mikes tactical duffel that I love too. It is 50L or so sized pack with a decent backpack strap set up and light padding with a rubberized bottom.
I’ve had three duffles of various sizes from Cabelas for years. They’ve suffered terrible abuse on many snowmobile and mountain expeditions and in a variety of trips to the third world. They’ve been immune to the rough handling of native guides, bored bush pilots, careless porters, and strapped to zodiacs, rickety third-world airplanes, tap-taps, and pickup trucks. Big fan.