How to upgrade my Rega Planet based system

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.

I plum forgot another great tweak for your system. Couple every component (speakers too) with Audio Points, including the rack you have your system on. If its wood you can get Audio Points that will thread into the legs of your rack. The Audio Points will be a sonic boost to your entire system. Call Robert, at Star Sound Technologies, and he will give you the scoop as to what size Audio Points you'll need. I'm a coupler. Not a decoupler. People mix the two. That's a crying shame. I don't even want to go there, because of the superfluous threads it'll generate. Email me if you, truly, want to be set free.
happy listening,
warren