Well I'll be... I was a little skeptical but call me a believer! When I was reading Lasse's thread over at PETT I was unsure if I have ever heard or could hear distortion from a P-Core inductor. I've used a few P-Cores in past projects and never gave it a second thought.
My prototype crossovers for the Piers project (Peerless MTM's) use a Jantzen 1.5 mH P-Core inductor unwound to 1.25 mH in the LP woofer filter. My package from Madisound arrived today. In it was a pair of their "Sledgehammer" 15 awg laminate core 1.25 mH inductors. They are $13.70 (about the same price PE sells the P-Core) and they have the same exact DCR as the P-cores, 0.12 ohms. So I soldered one lead in and use a jumper on the other lead so I can quickly switch between the P-Core and the Sledgehammer.
At about 80 dB the two inductors sound exactly the same to me. But when I increase the volume to about 95 dB there is an obvious difference. I'm not very well versed in audiophile "speak", but the Sledgehammer inductors sound cleaner and like the woofers aren't working as hard. My Rotel amp (class AB) is only rated for 90 wpc into 4 ohms, so much less than the P-Core's 400 watt rating.
This is obviously not double blind ABX testing and I might be biased since I just spent about $30 on the inductors. I am going to set up my OmniMic and take some FR and Distortion measurements with both coils at 80 dB and 95 dB and see if there is any difference.