keep_hope_alive is right on target, as usual.
Aside from the development of class D topology, amplifier tech hasn't really advanced in the past 20 years. Manufacturing has become more automated, so prices have dropped. And they've tended to pack more crap into the input stage of newer amps (EQ's, xovers, etc). But when discussing quality versus quality, there has been no new paradigm shift in tech advancement to really improve the amplification stages of amps today. This is why 'old school' amplifiers tend to still be so popular today, for the most part they are still just as useful/viable/legit as they were when they were new. Power handling on speakers has changed relatively radically in the past 10-20 years. But for subwoofers, not mids/tweets/comp sets. So a 50w x 4 or 100w x 4 amplifier of yesteryear is still just as effective a piece of equipment as it was when it was made. I prefer as much of my signal processing outboard and upstream of the amplifier as possible anyway. To me, the ideal amplifier would be a simple gainblock, voltage in one side, amplified voltage out the other. Nothing but gain adjusters.
More old school amplifiers were 'hand made in the USA' than amplifiers these days. That still means something to a lot of people. There a lot of points to be made on both sides of that debate. I was in the automation industry for many years, as a designer, so I have a particularly keen understanding of how automation can benefit a product. But I also have a nostalgic appreciation for the idea that a certain product was hand-made, by a trained person (or people), who took the time and care to make the product. But that is also just nostalgia in many ways too, because lets face it, plenty of stuff has been hand-made, and was still crappy.
Speakers. Speaker tech has evolved in the past couple of decades, and car audio speakers have become more refined in some ways (speaking very generally here). Like hope said, they have become more refined in ways like better quality baskets, better surround materials, and whether it improves sound quality aspects of the speaker, or simply aesthetic appeal of the speaker... their cones have been more intricate and visually appealing than older speakers.
To me, the biggest step in subwoofer tech in the past two decades is BL optimization, and general motor design improvements (like the use of shorting rings, better cooling, more linear excursion potential).
New speakers, old school amps, and I said old school subs only because I consider new subs to be unnecessarily designed to have a low efficiency just so they can claim higher power ratings.
Its not uncommon for new subs to be rated at low 80's decibels (1W 1 meter) where the old school subs were in the low to mid 90 decibels.
Old school amps need more efficient subs because they usually dont make 1,500 to 5,000w like the newer amps.
Hoffman's Iron Law tells us: efficiency, low frequency extension, small enclosure... pick two. The car audio industry has gone to lower efficiency drivers because over seas manufacturing has driven down the price of amplifiers, allowing the speaker makers to make subs that will work in smaller enclosures (important for the guy wanting to cram as many/big of speakers in his Corolla's trunk as possible) and still retain their low frequency extension.
The power handling of the subs has been increased to compensate for this paradigm shift in how speakers utilize HIL. This has been done by way of larger diameter coils, more use of flat wound, increased cooling from better frames and motor designs that maximize air flow through the gap, etc. Its not a direct function of a subwoofer being less efficient.
Us old schoolers remember back when putting 2 10's in your trunk took a massive box. And that 500 watt 'monster amp' you bought for it, cost more money than the 2500 watt amp you use today. Many of those amps that were originally marketed/purchased to run subwoofers, are now being used to drive front stage speakers these days.