Excellent sounding cartridges for Rega P3/24?

I wouldn't take a little Internet "buzz" or speculation based on the tonearm mass/compliance formula to rule out the Ortofon 2M Black. At the very least, talk to somebody at Needle Doctor or Acoustic Sounds. I believe there are several online vendors that sell both Ortofon and Rega products.

10 Hz may be the ideal resonant frequency, but the tonearm database matching grid seems to indicate that anything from 8 to 11 Hz or so is ideal. The 2M Black on an RB300 comes in at about 8 Hz.

And another thing: The formula may be able to give you the resonant frequency, but it can't tell you how strong the resonant frequency is, because that depends on other factors such as tonearm design and damping. I'm not sure how vulnerable the RB300 is to mass/compliance resonances.

There are certainly some designs that can dampen most of the resonance back down. For example, a cartridge's damper brush (such as is on the Shure M97xE and was on the V15 series) reduces the resonant hump by around 5 dB. Ditto for tonearms with fluid dampers. I'd rather have a 0 to 1dB resonant hump at 8 Hz than a 5dB hump at 10 Hz.

Fido:
I had also been looking at the Ortofon 2M black but then I read a post that said this cartridge is not a good match with the Rega tomearm.

If you check the tonearm chart for the Rega RB300, you'll see that it shows 8-11 Hz as the "good" range. The Ortofon 2M Black weighs 7.2g and you can figure about .5g for mounting screws. Its compliance is 22.

So if you look at the Resonant Frequency Evaluator, you'll see that an RB300 with 8g worth of cartridge and hardware, with a compliance of 22 (10-6 cm/Dyne) comes in just at 8Hz, which is theoretically in the "safe" zone.

Audiofeil, an experienced dealer and A-gon poster, places the ideal resonant frequency at 10 Hz, but that doesn't necessarily mean that 8 Hz is "bad." He did state that 7.23 Hz (which it would be on a Technics arm) would be too low. It looks like the 11.5g effective mass of the RB 300 is at the upper limit, and lighter arms such as the Hadcock and VPI JWM9 would line up with a 9-10Hz resonant frequency.

It's my understanding that the VPI JMW9 arm is compatible with the Rega armboard.