Rega planar 25/rb600 cartridge recommendations

Denon low compliance MC on Rega tonearm with 9-10g effective mass?
This is not good, you need a 30g effective mass tonearm for Denon 103 for optimal performance. 

You need a mid compliance MM/MI for this tonearm. 

Which Grace are you looking for ? 

@skipper320

Looking at a grace f9e

sold one last month, still have original styli.
In my opinion the Victor X1IIe surpass Grace F9e in everything.
Victor is a mid compliance MM with titanium pipe cantilever and elliptical tip. Grace compliance is higher and cantilever is aluminum (RS9e) with elliptical tip. Jico styli available for Victor X1II series, but original stylus is better (titanium pipe, tension wire). Sony XL50 with Boron Pipe cantilever is better than Grace F9e. Sony has its Cup-shaped damper and tension wire, the joint-less delta-type core and Super Elliptical tip. A tip mounting hole made using a laser beam. This technology is no longer available. If you need help with decent mm feel free to ask in PM.

Use the Denon and just add some mass to the tone arm

You can’t add 10-15g mass to the tonearm to make a low compliance cartridge like Denon 103 optimal for this lightweight modern tonearm. Even if you will add some mass you will have some problems with counterweight position, the counterweight is too light.

Also using a conical stylus in 2020 is anachronism, this cartridge designed in the 60’s for broadcast (radio). The life-span of the conical stylus is too short. It’s nearly 5 times shorter than MicroRidge for example. If you could use advanced MicroRidge stylus for 2500 hrs max, you can use conical only for 500 hrs max.

Rega tonearm designed for mid compliance cartridges. It’s much easier to find a better mid compliance cartridge (MM) with great stylus profile and extended frequency response than to mess around with low compliance Denon 103 from the 60’s on the wrong tonearm without detachable headshell.