I don't hang around the comps much...well, ever, but back in the day I did run across some LOUD (for the time) daily drivers. 4 Orion XTRs in a 6th order bandpass wall in a Pathfinder running off a single 2100HCCA. The shop ran a sine sweep on that one that was knocking stuff off the walls in the shop. That was in '92. Friend of mine in HS (still early 90s) had 4 Kicker C12s in a wall running off a Linear Power 2202. You knew when he got to school. All the lockers on campus rattled. My setup in college was pretty loud, as well. 6 10W6s on a 2100HCCA in a cutout in a reg cab Ranger.
None of these would be considered loud by a lot of people on this forum, but considering the time period (early to mid 90s) and how expensive amps were (1kW was going to run you well over $1k) and that none of them were running more than a kW of power they were loud. You wouldn't be able to get that with any of the supersubs today on that little bit of power. It cracks me up hearing people say that 1kW per sub isn't enough.
The newcomers to the game are spoiled. They can get a fairly reliable amp that will do over 1kW for around $200. That's what I paid for my very first amp. It was a 30x2 entry level Orion Cobalt. 120W for the sub, a 10W1. Orion high freq drivers in the doors, amp running in tri-mode to power everything. Did all the install work myself and it sounded good. Not at all loud but that isn't what I was going for. I had a budget system and including the deck (a basic Panasonic CD player) I had close to a grand invested. For that kind of money currently you are looking at mulitple subs and amps and close to to 1000W. Efficiency isn't much of a consideration. When you really look back at what was done back in the day as far as SPL goes and some of the amazing systems that were built without the benefit of Class D amplification or cheap power in general.