What speakers for horizontal tri-amp configuration

What speakers are recommended for a horizontal tri-amp configuration? Assume 3 external tube amps (150W ea.) using the passive crossovers included in the recommended speakers. Speakers need to have 3 sets of posts (Bass, Tweeter, Mid).

I have seen sonic benefits from horizontal bi-amping. I also enjoy the flexibility in tube rolling to match the amp to the speaker being driven (tweeter, mid, bass).