Difference between New and Old Amp WPC ratings....

I own a pair of CV D-9's (I know this is an audiophile website, be kind, it's what I can afford for big sound). My question is this. Old amps, Marantz and the like have power ratings of 20, 30wpc, and newer amps have 70-150wpc. My CV's say they can take 300 wpc, and I have them on a Pioneer that is about 80wpc, and it gets pretty crappy as the volume goes up. Is there something about the wpc ratings of new and old amps that I need to know, or do I just need to get a PA amp to run these things?
You should try a tube amp on your big honkin' efficient Cerwin Vega speakers; you will be very surprised.
There are 2 issues here. The first is power ratings generally. Quality amps old or new generally quote real world power ratings. Most of the current Japanese based stuff inflate their power ratings by quoting a figure at a single frequency, so their real world rating is probably less than half that quoted.

The other issue is amp quality generally. Those with a poor power supply will play loud but distortion increases dramatically as the volume goes up. Clearly that is the case with your Pioneer. If you like it loud you just need a better quality amp, not necessarily a more powerful one.

Finally, newer amps seem to offer more power generally, likely because the cost of producing power amp components is dropping plus many are moving to class D which is considerably cheaper to build. LIke most things, each year the claimed power increases to compete with other brands. Just like horsepower in cars.
Yes there is a difference with power ratings with old amps and new, if of course you are talking 70's amps or even reciever. From wikipedia, In the US on May 3, 1974, the Amplifier Rule CFR 16 Part 432 (39 FR 15387) [3] was instated by the Federal Trade Commission (FTC) requiring audio power and distortion ratings for home entertainment equipment to be measured in a defined manner with power stated in RMS terms. This rule was amended in 1998 to cover self-powered speakers such as are commonly used with personal computers.

Before this rule less = more by todays standards.
I have a Pioneer SX 626 rated at 27 wpc, by today's standards it's like 3 or 4 times that
Actually, I believe (not 100% sure) that there was another rule change subsequent to 1998. It actually had the effect of going the other way as IIRC it reduced the preconditioning stress and discoutaged and/or disallowed the power RMS @ X% THD over bandwidth spec that had been traditional. Someone with more energy might want to research it further, but the current specification regime is I believe less useful than previous.

Apologies in advance if I screwed this one up, but I'm pretty sure there's a kernel of truth in here smoewhere.

Not all wpc are created equally. Some amps, not necessarily new, sound significantly more powerful than others of the same power rating. I have heard the class A Bedini 25X25 sounds huge for a 25wpc amp. The classic NAD 3020 kicked out an amazing 20wpc. And tube amps can pull off the same trick.
Me thinks there is much more to the equation than the wpc rating, whether using the old or new standard.