Of the few pre's I've tried with a Passion, I particularly enjoyed the Passion & the CAT Ultimate -- both of these are expensive, as you note. I also liked a Symphonic Line -- but that's even more expensive...
From there it's pure speculation; the Passion stereo is an easy load, so the choice is huge. It's also an exquisite amplifier with excellent resolution. So, using a "good" pre (as opposed to an "outstanding" pre) will be audible.
Have you thought of going passive? A transformer volume control can offer excellent results, and you have lots of power in the Passion and reasonably efficient speakers -- so maybe you can live without the extra db's a pre will offer.
From there it's pure speculation; the Passion stereo is an easy load, so the choice is huge. It's also an exquisite amplifier with excellent resolution. So, using a "good" pre (as opposed to an "outstanding" pre) will be audible.
Have you thought of going passive? A transformer volume control can offer excellent results, and you have lots of power in the Passion and reasonably efficient speakers -- so maybe you can live without the extra db's a pre will offer.