MC cartridge that best mates with RB300 arm....

dynavector mc carts are perfect for rega, imho.

I second the Dynavector recommendation. Absolutely musical in an RB 300. Available in high & low outputs - but some say the low outputs sound a little better, if you have the correct preamp for them. I have no real experience in that, I've only used the low outputs, and have been very pleased. Dynavector is one of the true bargains in Hi-fi. Check out the info at:
http://dynavector-systems.com/dynavector/
A 20X-H or 20X-L should be magical for you.
Good Luck!
Ed

The dynavector 20X-H or -L, depending on your phono stage. The lower output MC's can be very demanding on the front end, and on the tt/arm. The 20x-H should work on a MM front end, due to its high output. I am not sure a the rega 3 and 300 arm can handle a low output without an Origin modification to the 300 arm.

AT OC-9.

If you go DV-20x, get the high output. Getting the LO is pointless with the P3/300, and a bit risky, sonically. Personally i'd get the 10x4, and possibly put the rest towards a Heavyweight or save it to partially pay for a TT or wire upgrade. Good luck.