Thinking about new cartridge

.5mV is a-bit bellow the bottom line for your phono sencitivity. thus you should have a preamp with input sencitivity at least 220mV or better.
if you have a unity-gain or passive preamp I would not recommend you to use lower than 0.8mV cartridge certainly depending on your power amp sencitivity as well.
the cartridges you've mentioned are all great performers but for most of them you might need a step-up transformer thus you can even use mm position in your BAT.
also you might consider upgrade of the phono as well to pass X ono used, audio synthesis... that have a max gain arround 70dB suitable to drive even helikon sl with no help of step-up transformer.

I don't think it is enough gain. I have a Shelter 501 with .4mv and my MFA Magus preamp has 66db of gain, and if I plug straight in to the preamp, without using my Cotter Step-up transformer, then I have to turn up my volume to full to get even marginal listening levels.

You might have slightly different results, but I think that is is too marginal to risk that. Too high a probablility that your volume level will be too low.

I think that if you like the low-output cartridges, like I do, you should get a 20db step-up transformer and use it in line with your phono cable. That's what I do. And you don't lose any signal with a decent transformer. You are dealing with miniscule signal levels which are not going to saturate any transformer. And the transformer also provides a better impedance coupling than going straight in. Kondo Audio Note-Japan, states that they recommend using a low gain phono section and using a step-up transformer for this reason. I use a Cotter, and I am very satisfied, and my system is so ultra-revealing that I can hear if a bug farted in the recording studio. On some recordings, I can hear the master tape deck running.

So, you are not going to lose anything by doing this, and you can gain cartridge choice flexibility, and try out some of those low output MC carts you've been hearing about. You don't have to change preamps, or dump any of your other equipment. If you do change preamps, you can keep the step-up transformer, because it is a separate box.

Total flexibility, and great sound, and improved selection range of cartridges. Sounds like a good idea to me.

Also, the 309 will handle all the cartridges you mentioned.

The Denon is slightly lower output (mine is .29 mv) but
is a superb bargain at $298 through Denon Electronics USA.
I'm using mine in an SME IV and it is a terrific match.

You don't indicate what other equipment you own, or what kind of "sound" you want in a cartridge, so it's a bit difficult to know what to suggest to you. There are two cartridges, however, that I think ought to be on your audition list:
1. Grado "Reference" (0.5 mV output version) ($1200);
2. Dynavector 17D ($750).

The Grado is a moving iron, and has a wonderfully rich, musical quality to it. The Dynavector has been around for a number of years, but is one of the best moving coil designs, and it gives a number of much more expensive cartridges a real run for their money.