Steve, Sdcampbell hit that one right on the head. The MkIV upgrade is VERY significant. I also use the RB900 arm. If you can afford the MkIV upgrade, do it. Your stock RB300 arm may fall slightly short after the upgrade.
Although, your 300 arm can be upgraded quite nicely. See below:
http://www.tonearm.co.uk/1-rega-mod-modification.htm
OR........
http://www.expressimoaudio.com/cart/shop.html#
The RB900 arm is a very nice arm. But, it is even better with some of the mods such as VTA adjuster, Heavyweight counterweight and internal wiring. All those same mods can be performed to your 300 arm (which you already own). If you ever decide to upgrade to the 900 arm, you could carry over the mod. parts from the 300.
Best of luck, Ed.
Although, your 300 arm can be upgraded quite nicely. See below:
http://www.tonearm.co.uk/1-rega-mod-modification.htm
OR........
http://www.expressimoaudio.com/cart/shop.html#
The RB900 arm is a very nice arm. But, it is even better with some of the mods such as VTA adjuster, Heavyweight counterweight and internal wiring. All those same mods can be performed to your 300 arm (which you already own). If you ever decide to upgrade to the 900 arm, you could carry over the mod. parts from the 300.
Best of luck, Ed.