Originally Posted by Speakerphile
Keep in mind though that they may not "rate" it to 4-ohms for a reason. It may cause it to run outside it's desgnated operating temp, which may or may not cause issues with lifespan. It will at the very least cause it to suffer from poor sound quality with less power in reserve for those nice explosions! Though I would really recommend a real amp for your ML's.
Many amps and receivers that aren't rated for 4 ohm use are simply rated that way due to the fact that, while they can support most 4 ohm speakers, they wouldn't be able to pass the 4 ohm testing to get UL certification without extensive extra engineering to keep the temperatures down, or other issues. I know I've used a Marantz amp that was only rated down to 6 ohm for this reason, though they told me they used the exact same amp to drive their Snell speakers in their listening room, which are 4 ohm designs.
That said, planar and electrostat speakers drive really hard 4 ohm loads typically, much harder loads than most receivers can handle. If I was driving those, I'd get something like an XPA-3 from Emotiva to handle the front 3 at the minimum, as even if you still have to drive rears with the receiver, it'll be much easier to do since it won't have to drive those front speakers as well. However, as the Pioneer seems to lack a 12V trigger, you'd need to find a way to turn the amp off and on manually, or just leave it on all the time (I do this with my UPA-5 from Emotiva).