what tube preamps would mate well with the Revel F208s and A21?Unfortunately, I believe the A21 has a 33k input impedance. Most tube pre’s would have a rolled off low end driving that impedance.
You would need to look closely at the coupling caps the pre under consideration is built with.
Though the pre's output impedance will play a role, you would probably want at least 2uf minimum, that would get you to about 25hz.