Ok guys, I know I am throwing myself under the bus here, and I know that some of what I say isn't going to jive with everyone.
First, I used "should" 4 times:
The first and second times are so you don't hurt yourself/others. Hearing loss is a real danger in our business. Notice how the second situation is from the perspective of someone in the audience trying to order a beverage at a small bar/club. Most of these venues are not intended to be ear splitting power domes, they are places people go with friends for entertainment and/or food, beverages, conversation; a night out.
It seems slightly ironic that I am being told "don't tell me what my rig should be" or that I don't understand the need for more power, when the third and forth times "should" gets mentioned, I am advocating for more power.
I know that there are situations where you need more power. I know that some genres "require" more stage volume than others. As with a lot of talkbass, context matters and often is overlooked. This is why I specified venue sizes, I strongly advocated for PA support, and I said that sometimes you do need more power.
I am offering advice (that still holds ground) based on "average" situations as the thread stared off as "I'm a relative newbie bass player. I'm a little confused as to how much amp power you need for the following situations" A lot of bassists (myself included) aspire to owning gear that they want, but isn't absolutely necessary, nor would it be recommended to a new bassist. I would love to have a Fodera, Alembic, Conklin, or some other high end bass, and a wall of cabinets powered by an atom bomb. I don't think this is a need for most people, nor would I recommend it to a new player. If you want or own gear like this, all the power (literally in some cases) to you. I repeat, most new players are looking for gear that will work, not a "if money/carrying cabinets wasn't an issue" solution right off the bat. Some of them might eventually want/need more that 400-500 watts. Most bassists that get to that point in their career have technically and musically advanced/matured, and want to upgrade/something different than what they started with anyway.
I have one "big amp". It is a 450 watt head with a 4x10 cab. I have played shows with extension speakers, and I have played shows where I use a larger setup. I love me an Ampeg fridge like everyone else. I can do almost everything I currently do or can see myself doing with that amp. I have a couple of smaller combos that come out for jazz gigs or practice situations. I might find myself in a situation where I need/want more power (I currently rent or use the backline provided by the pro audio company in these situations) and if I consistently find myself there, I will consider something bigger.
The "one amp with lots of head room just in case" scenario does make sense, and I am not knocking it entirely. I just don't want to have to carry that much rig all the time when it will only make it past a master volume of "think about turning it on" in rare circumstances. Personally if I am playing a show where I need more power than I've got, I am aware of it well in advance and will deal with it accordingly by renting or using the provided backline previously mentioned. If that idea doesn't work for you, renting in your area isn't an option, or you really like having a ton of power in your own gear, that is your own choice.
If you want or feel you need more power than this in the situations where I suggested 400-500 watts would be enough stand alone power (rehearsals and <100 capacity gigs) I am not stopping you. I am not trying to advocate "less is more" but trying to suggest realistic parameters for people who are considering that first step past a 50 watt bedroom practice amp. If you are in the "more is more" camp, I respect that. I just won't be dropping by your trailer for a beer any time soon.