Best all-around cartridge for 500.00 new

I too vote for Dynavector 20XL if your phono can drive it. If your phono can't drive it you can get 20XH. They're great rock-n-roll and fussion cartridges.
Grado most-likely will hum on Rega.

A fellow Audiogonian told me that Denon DL 103D's, a classic of yesteryear, are still available, and at only $229 plus $5 shipping. I had one decades ago and loved it, but of course your phono stage would have to work with a roughly .3mv MC. If you're interested, the only way I know to contact Denon is by telephone: 310-974-1010. It takes a while to reach a human being, but once you do, they're polite and helpful.

I've ordered one for a Rega 25 system but don't have it yet. Grados used to hum with Rega tables--has that changed?

Let me suggest the Clear Audio Aurum Beta S MK 2. I have recently purchased this cartridge (actually its predecessor) and I am very impressed.

Tom, i recently picked up a Denon that had never been used for $100. One of my buddies found it and a few other cartridges from a vinyl "addict" overseas and snagged it for me. He had used one previously and thought it was a phenomenal bargain for the money. As you mentioned though, it is a low output cartridge and does require more gain to perform optimally.

If you're looking for a high output MM type, try a Stanton 881S Mk II. It will smoke a Shure in terms of overall reproduction and musicality and costs measurably less. All the V15 type cartridges suffer from a non-linear distortion problem that an honest reviewer would mention ( some do ). The Stanton has a much shorter and more rigid cantilever. This results in faster tip speed, more detail due to a shorter path to the motor assembly and less distortion due to the cantilever not flexing. The frequency response is more neutral and there is less "grundge" in the upper mids and treble without sounding near as "closed in". The 881 will also track BETTER than the Shure on high amplitude passages with a lot of vertical deflection. A good source for the 881S Mk II is Kevin at KAB Electro-Acoustics. Sean
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I am currently using a Grado Sonata (low output version, less prone to hum) with my Linn LP12 and find it great with vocals and acoustic music. However, it is not the most detailed or dynamic. If you can stretch your budget or get a discount buy a Benz Glider as this cartridge does almost everything quite well. I've heard and read many good things about the Denon (as recommended in the above post) and Shelter cartridges. I understand why you said "new" as buying a used cartridge is an iffy proposition.