There are no headphones under $100 that a serious audiophile would accept.
That being said, I noticed that you've been looking at on-ear headphones. I've owned these for over 15 years:
Amazon.com: Sony MDR-V6 Monitor Series Headphones with CCAW Voice Coil: Electronics
I don't think they fulfill the serious audiophile's ear, but I use them with my Zune 30 and I think they sound very good. I eventually had to replace the earpads with velour pads, as they WILL eventually disintegrate. Actually, I think velour pads actually make them sound better.
I also own the Sennheiser PX 100's, and the Sony's are a significant improvement over them. The PX 100's sound hollow, in comparison. EDIT: These are studio headphones, but I've never felt the need to use an amp on them - I'm using them right now at 50% volume, and it suits me just fine. I have a FiiO E5 amp, but I only use that on my Gigabeat.