@rwpritchett I posted a ton of pads on HE-6. You can extrapolate to see the relative differences and how they might fair on HE-500. I think MrSpeakers Ether C pads and Dekoni DT Series pads would work well.
Audeze Vegan pads (Discontinued) These have very good comfort as they are more like soft pillows than any other pad. They also have an excellent bass to mid response. The upper mids and treble is where I personally find issue. After the dip at 2kHz they start rising up and the treble can get too fatiguing depending on personal preferences and associated gear.
Brainwavz Angled Lamb Skin pads. I like these quite a bit. They are angled so they have better imaging and sound stage depth. Like other pleather or leather pads they have better bass conduction than the velours, even the Audeze Vegan pads. They are relatively well behaved throughout the frequency response. My only gripes are the 1kHz bump and the small 10kHz peak.
Brainwavz Hybrid Memory Foam pads. These are similar to the Vegan pads. They would be my recommendation if you want that sound and can't find the Vegan pads. They do offer a more immediate sound as your ears are closer to the drivers compared to the Vegan pads. They have a fun V shaped sound to them but they aren't natural sounding.
Dekoni DT Series Sheepskin pads. These are one of my top picks. They have a forward mid range and lower treble, great for guitar and a more present vocal presentation. They have a good balance with the bass and treble and are one of the few pads that seem to tame the 10kHz peak. They also present a nice sound stage as the pads have some depth to them but focus the sound well.
Dekoni TH-900 Perforated Sheepskin pads. These are a good alternative if you like the stock velour sound but want less high treble energy. I didn't like the way the perforated material felt on my skin but others might not have an issue. This is a pretty agreeable frequency response but a bit bass light for my tastes.
Dekoni TH-900 Sheepskin pads. These are the same as above without the perforations. They have good bass and a more laid back presence region compared to the stock velours. They are similar to the angled Brainwavz in sound but are a little brighter overall. They have a slight angle to the pads but not to the same degree as the angled Brainwavz.
Hifiman Focus A pads. I'm not a huge fan of either focus pads but these are the better of the two. They have a little more natural presentation with less 4kHz energy. The build quality is atrocious on these, I've had one pair come undone at the mounting ring.
Hifiman Focus pads. These are the more energetic version of the two pads. They are great if you like edgy in your face rock guitars or exciting percussion. They don't have the best bass quantity or quality compared to the other pads.
Hifiman stock pleather pads. These aren't that bad but they don't breathe well and the laid back presence region combined with the 10kHz spike makes almost all other pads an improvement.
Hifiman stock velour pads. I'm one of the few who really like the stock velour pads. I think they get the bass to mid transition perfect and only suffer because of the excess treble energy. Some creative front damping can make these the preferred pad for me and possibly others.
MrSpeakers Ether C pads. These are my top choice for my preferences. They are angled slightly, keep your ears close to the driver for good immediacy of sound, and excellent balance between the bass, mids, and treble.
ZMF Perforated Ori pad. This is actually a pretty good pad. If you like the focus pads but want more bass try these pads. They have good presence but with supporting bass so they don't become overly bright.
ZMF Suede pad. Zach was really curious to see how these compared to the Audeze Vegan pads. They are more similar than not but the Audeze Vegan pads have less energy around 1kHz. This is do to different damping such as material and foam but also their physical size, as they are at least 1.5 times bigger and don't fit as well as the ZMF Suede pads on the HE-6. I personally prefer Leather or pleather pads over Suede and velour pads as the bass is usually better with less treble harshness.
If you have a pad that you want to get measured I would be happy to do so. However, I will not purchase pads just for measurements.