Hi-
I'd replace the Valhallah with a LIngo (or Naim Armageddon) Power Supply, which will _really_ improve the bass. The Circus helps a bit to control the bottom end, but doesn't really extend it. Many Circus upgrade owners were actually a little disappointed with it, but I think it's a worthwhile addition - after a Lingo. I would also recommend a thorough set-up by a Linn expert. It makes a tremendous difference in performance if the suspension is "in tune."
As for a cartridge, I'd heartily recommend a Dynavector. They work GREAT with a Linn, and you just need to pick a model that matches your Preamp's input. That will certainly add the clarity you're looking for - a definite step up from the BPS. E-mail me if you would like a specific recommendation, and Good Luck!
Ed