I didn't read the entire thread but the answer is "it depends":
1. What is your situation, is it a band? With a loud drummer? Loud guitar player? More than one guitar player?
2. What are the venues you play, smallish? Mid-sized rooms, a lot of large rooms or outside gigs?
3. What style of music? This makes a big difference.
4. 4 string or 5 string bass? Generally you needs more with a low B string
I've been out of playing live for a while but what met my needs in the last band - Classic Rock cover band with an extremely loud drummer and one loud guitar player
A. Inside - less than 100 seats or so: 350 - 450 watts through a single 2 x 10" cabinet. This was ALWAYS Plenty for those kinds of rooms. If the room was small I used my Ampeg B2-R (350 watts); slightly larger I used a Bugera BVP-5500 (rated at 550 watts but more like 450, it was louder than the B2-R. The cabinet in question is a Carvin Redline 2 x 10", this is not on par with the boutique stuff but was light (42 pounds) in the era when I purchased them (I own 2) and sounded fine to me, my bandmates and the audience.
Not a great low B cabinet but I play four string basses.
B. Outside - generally I used a rackmount rig with an Ampeg SVP-Pro tube preamp, which fed a QSC PLX-1602.
This was used mainly with two Carvin cabinets, one of the 2 x 10" cabinets on top of a single 18 cabinet.
The cabs are four ohms so I got 1,000 watts from that rig, NEVER felt like I needed more and generally never cracked more than about 4 on the volume output of the Ampeg preamp.
Sometimes I used a pair of the 2 x 10" cabinets but I preferred the 2 x 10" plus single 18" setup.
Note these situations for the larger gigs always had PA support as well.
I used the "big rig" from stuff like opening for a version of "Asia" in a venue that held about 600 people to playing outdoor festivals, one time we headlined a 4th of July thing for a good 5,000 people.
So my simplistic answer(s):
300 to 500 watts into a single cabinet for smaller venues or venues with really good PA support
800 to 1500 watts into two cabinets for large shows, especially outdoors. A 300 watt SVT will handle this just fine as well but at the cost of lugging around an 85 pound head and a 160 pound cabinet.
My Power Amp/Preamp rack (which also has a PSU/volt-Amp meter, a rackmount wireless and a Korg Rackmount tuner) weighs about 48 pounds. The Carvins are light, 42 pounds for the 2 x 10", 72 pounds for the single 18".
I use the factory casters on the single 18", and I would just set up the entire rig on the side of the stage and then roll it where I needed it.
Even with the rack front/rear panels, and covers for the cabinets I could set up or tear down that rig in under 5 minutes....easy peasy.
Analogeezer