How to upgrade my Rega Planet based system

All of the above; a subwoofer, power cables, and conditioning.

Buy REALLY expensive power cords. Like Siltech Compass Lake. Your dealer will be glad you did.

Subwoofer is the way to go. You'll be set free...I love my Sunfire Artchitectural Subwoofer. 10" single front firing, made for 2 channel listening. It's a shame you don't hear about this baby enough on the 'gon. It's fast, extended, tight, and integrates beautifully with my M 20s. It lists for $1400, but can be found nicely discounted (new) for less than $1400. It's an 11" (45 lb.) cube with a 1200 watt amp. (Sunfire also has the Architectural Signature, which is a 12") If space is a factor, this may do it for you. I compared it to the Velodynes and the comparable Rel.

Isobearings under the Planet with tighten and/or extend bass reach and generally clean up the attack. Agree with all the other responses.

A caveat on the sub, though. A good sub will improve bass a lot, and arguably also improve soundstaging if well set up, but that's it. With power conditioning, the whole picture/spectrum is improved. As a matter of priority, I would do the sub after power tweaks. Hey, it's free advice.