Usually all the theory says vertical is best, assuming identical amps. Few things are changing though:
- Interconnect and speaker cables
- Apparent amp input impedance is cut in half.
Normally with solid state preamps the latter doesn't matter. Cutting from 25kOhms to 12.5kOhms is usually well above a pre's output impedance, but if the pre is tube or just high output impedance, or the amp is low input impedance it could be enough.