As long as amps come with volume controls, you never have to worry about having too much. From an acquisition stand point, there isn’t to much difference in price to get from barely enough to get by to overkill. 400 watt powering a fairly efficient cab(s) can cover a lot of applications.
Example, I covered a room with such a rig seating 700 people with no FOH support on a gig with an 11 piece Motown review band with horns. Same band with FOH support, I use 300 watts powering a 112 cab as a personal monitor.
On the other hand, if you are playing doom rock with a couple of guitarist with each with 100 watt Marshall stacks and a drummer that plays with axe handles, you may need to revisit the 400 watt one cab thing or pray for FOH support.
You're right about the volume control with one caveat that was driven home to me in spades when I moved to Washington state in 2015.
I found myself playing in two groups. One, a 5-pc. rock group with bass, drums, 2 guitarists, and a keys player. Keys player was the female vocalist. The BL sung most of the songs and I sang a couple per set too. Second group was a Blues/Rock Trio.
I had received my DNS-410 cab at my home in Virginia just about 3 days before we loaded everything up and moved just under 3,000-miles away. That cab handles 1400-watts with a max SPL of 137db. Sound is clean and super clear and if you like to play bass chords, you will very distinctly hear all three notes. It's a very LOUD cab and doesn't take a lot to get it up to gig levels. I would feed it with my 800-watt Eden amp bridged into it at 800-watts. I figured I had bought the last cab I'd ever need to buy because, you know...that volume knob works just fine. Man was I in for a surprise.
The 5-pc group had a standing gig on Friday nights from 7 to 10PM at a small brewpub. It would hold about 60-people or so indoors and if they opened the patio outside, that would go up to a couple of hundred. I rolled my DNS-410 into the brewpub and the owner's eyes got as big as silver dollars. He said, "That's way too much for here. Use that Peavey 112 combo over there.
Although that Peavey was loud enough for that small room, I ever could get that little 112 combo to sound good. I ended up buying an Eden TN226 (225-watts rms) and a 4Ω Eden EX-112 for small venues and that's a great and very light set-up for small venues of say, 100-people or less. It sounds great and it's enough power for those small venues (we have over 400 brewpubs in the stated of Washington!)
So you are absolutely right @dbsfgyd1, with judicious use of your Master volume control, one cab is all it takes. And, my DNA cabs just like my Eden cabs, sound great throughout their volume range from soft to LOUD! But you don't fight with the owner of a place if you want to come back. Since we played that place over 40 Friday Nights per year hosting their Open Mic night, it was important not to piss off the owner.
You're also spot on about 400-watts handling a lot venues. My DNA-800 amp puts out 400-watts into my 8Ω DNS-210 cab and will put out 800-watts into a 4Ω load. I've found that with just 400-watts, I can get plenty loud enough for most of my venues. I even use my DNA-800 amp putting 400-watts into my 410 cab when playing outdoors within a confined space (lots of reflections). But for a wide open Music Festival I'll rbeak out my Eden WT500/800 and bridge it at 800-watts into my 410 for that. With that rig I get told to turn down if I run my master up to 10:30, so I set it at 10:15 and thank Eden and DNA that I don't need to lug around an 810 cab.
