Which cartridge should I upgrade to now?

Get the Shelter retipped and save your money to buy a better tonearm.

That arm is a perfect match for this particular Nottingham. Big upgrade would be to Nottingham Spacedeck/Spacearm but that's big cost too.
You didn't mention your phono stage. Also, I understand that you consider both MM and MC cartridges. $500 for new MC is tough.

Only interested in MC. I'm in Sydney. As I mentioned, Denon DL 103R can be gotten for $330 out here.

Phono stage is a Lehmann Black Cube with external psu. That could be upgraded too, & the table, but funds don't permit that at this stage.

Re getting the Shelter retipped, that means shipping to the US. Not impossible but lotta mucking around. Who does the retipping btw?

In the USA I have used Soundsmith and Phono Cartridge Retipping both with excellent results. In England, Expert Stylus does retipping, with good reviews, but I don't have personal experience. Just type cartridge retipping in google or Audiogon for additional comments and their websites.

That Rega tonearm is good for the money, but will not let you achieve optimal performance from your phono cartridge.

Ok thanks for the info. Re tonearms, what do suggest as a replacement for the RB250? Not in a position to spend four figures on an arm. Second hand is probably the go for me.

I don't think anything you're talking about will be as good as the Shelter.  The Denons are OK, but that's about it.  Personally, in the budget range, I think the Goldring Eroica is very good and I know it works well in Rega arms.  Good luck. 

Garrott Brothers retipping service has been owned and operated since 1992 by Audio Dynamics in Melbourne, AU.
You can also facilitate a Garrison retip through Decibel Hi-Fi. The linked page includes their retipping rates, which are well within your budget and give you back your cartridge worth around $1700 AU.

I would not replace a Shelter 501 with a Denon 103 AND the 103 is not a good cartridge for that light to med weight tone arm. A much better choice would be a Dynavector 10X5 MC (2.5mv) for that arm, IMO. Lots of jump factor with this cartridge, you cant go wrong. Keep your tonearm and change to a proper cartridge is my advice. Also, I agree re tipping the Shelter might be the best option IF your changing your tonearm (only use Soundsmith for that, imo), but that Dynavector is really nice with that arm.

Matt M

Ortofon MC Quintet Blue

Ortofon 2M Bronze MM is also excellent

I don't know what the Dynavector costs in Australia; but in the USA it's a bit over your budget. I think the Denon DL-301 MKII is a better match for your RB250 than the 103R. I really like the sound of mine on a Sumiko MMT; but it's my first MC, so what do I know.

I think re-tipping the 501 would be a tad cheaper, unless you don't like it's SQ. In that case you could sell it and add the cash to your budget and get the Dynavector or maybe even something a bit better.

You should consider an Audiomods tonearm, which would be in your budget and is a drop in replacement for your Rega

I second Dynavector.  A much better match for Rega arms.  If you can stretch a little to the 20x2, you'd have your choice between low output and high output MC versions.

For consideration: I've seen many RB300 and RB600 arms come and go here on A'gon for well under $1000...would be a drop in replacement/upgrade for your RB250.

FWIW, I also agree that the Denon 103 would be out-of-sorts on a Rega arm. Because of it's stiff compliance, it really thrives with a much heavier arm.

I'll second the recommendation for the Audiomods tonearm. I upgraded from a RB250 to the Audiomods and have been extremely pleased with the improvements. One of the great audio bargains IMO.  

Got the Audiomods Classic 11 arm. Great. Improvements appeared subtle at first, more apparent with a bit of time. Not long after had a little accident with my Shelter 501 mk 11 cartridge. Found a just retipped one on evil bay. It's just bliss now, for me anyway. 

Thanks for all your suggestions

Glad to hear it all worked out for you. When you have time and budget you should send your old Shelter 501 to be retipped and you will have a spare.

Houseofhits - lots of good advice, but here's some additional info related to a couple of the above posts

Option 1: A very good "Denon" match for the Rega/Audiomods arm is the Denon 103 or 103R from ZU. - they offer a a couple of different "grades" of 103(r) cartridge based on channel tolerances and the aluminum shell gives it the added mass required.

Option 2: Having said that - send your existing cartridge to Soundsmith for cantilever replacement + stylus retip. They offer several stylus profile and cantilever options.

A word of caution; the more "advanced the stylus profile - the more important it is to have the cartridge setup perfect.

Do not use the printable protractors because they are not accurate enough - I used the Mint Best protractor - but there are others available that are comparable.
http://mintlp.com/best/9-series.htm

I have the stock 103 and also purchased a re-tipped 103 from Soundsmith - the retip’d version was significantly better across the board.

I epoxied a brass shim to both cartridges to increase the mass - it worked wonders...
http://image99.net/blog/files/23c020f75290d3392577113371f4dc94-38.html

Does your arm have the KLE Innovations RCA’s? If not try them. I have the Absolute Harmony RCA’s - They are superb at conveying the delicate signals. But they do require some burn-in.
http://image99.net/blog/files/a8494e8645dea3d750d109f819103099-61.html

BTW - I also have the Series II Audiomods arm - fantastic!!!  :-)

Regards - Steve.