"How can the Ohm Walsh 5000 match such a wide range of room sizes?"
4 separate 3-way level adjustments including a low bass level adjustment that you set to small, medium or large depending on room size.
"Can you play the Ohm's at lower sound levels and still get involving sounds? Some speakers sound flat and dull unless you turn them up loud."
Yes, but you need a good, high current, matching amp to deliver truly balanced results at low levels in particular.