Are there any suspension springs that will need adjusting if you add weight to the subchassis?
Alex - thanks for clarifying. I will attend to similar tweaks to my Heybrook when I have some free time (swamped with work at present). Not sure where you can get a headshell shim/weight for a Rega arm. Your local audio shop might be able to set that up for you. I'm sure a web search will turn up something. First step would be to figure out how much weight to actually add, so here is a good place to start: https://www.vinylengine.com/cartridg..._evaluator.php
It's the cartridge resonance evaluator -- plug in your tonearm effective mass (11.5g for the RB300), then cross-reference cart. compliance value and cart. mass (with fasteners) for the Denon, and see where you land. If in the yellow or orange zones, then a tweak would be in order. Calculate from there how much extra mass will bump you into the green. Easy-peasy. Any questions, let me know...
Best,
Svend