Best cartridge under $450? No Grados please

There is no such thing as "best".

From my personal experience I would recommend the Goldring G1042 MM cartridge. It has a 6.5mV output & it really sounds excellent. Your present phono stage should work just fine with the G1042.
Here's their website:-
http://www.goldring.co.uk/

They also offer various MC cartridges such as the Eroica H (2.5mV output), which is also supposed to be a good buy.
FWIW. IMHO.

I ran the Shure on my Rega P3 (which I'm guessing is what you have)for two years and replaced it with the Dyna about two months ago. You can click on my system link to see what kind of gear I'm using, and it will be clear that my priorities are not neutrality or transparancy, but rather a rich, smooth, enjoyable presentation. In fact, my system isn't really resolving enough to highlight dramatic differences between cables and such. Not surprisingly then, I didn't find huge differences between the two, though I ended up staying with the Dyna. Here's my assessment, then:

The Dyna's bass seems a bit flabby in comparison to the Shure. The Dyna has a richer midrange and more "sparkely" highs. Both are very quiet, with maybe a slight edge to the Dyna. The Shure is a better tracker, though the Dyna is no slouch in this area. The stylus of the Shure seemed to attract gunk more readily than does the Dyna. The Dyna seemed more sensitive to VTA than the Shure. The Shure has a user-replacable stylus and is easier to mount to the arm. The Dyna is better looking.

In summary, I found the Shure to be more lean/transparent, while the Dyna seem more lively. I liked them both and could have easily lived with the Shure for a couple more years.

Like so many things, I suppose, the right choice depends on what you're looking for. Good luck and have fun.

I'm a big fan of the Shure V15, which of all the cartridges I've tried, mm and mc, over 20 years, seems to get to the heart of the music and accurately preserve the rhythmic structure better than any other (the two factors are connected). It's also quite detailed. If you only want to enjoy the music without audiophile pretensions, this is the cartridge to get. Others may be more detailed or more dynamic (the Shure is no slouch, however), but the Shure simply sounds "right," you can relax. When you get the itch to improve it, a small dab (and I mean small, a tiny bead) of epoxy-resin here and there (one on each side and one in the middle: it can then be undone when replacement time comes) to hold the removable stylus assembly in place will squeeze some more performance out of it.

Let me put in another vote for the Dyna - incredible cartridge for the price. In my system the Shure did not touch it. Caveat: I used the 10x4, the predecessor to the 10x5.

Patrick