What makes a double bass "professional" grade?

[Eric Hochberg]

Sound, construction, materials and playability have to be there to suit a professional who relies on an instrument to make a living. Poor sound, no one wants you; poor construction and/or materials, the bass might not work the next day; poor playability, you won't be able to do the job day after day.

Regarding plywood, there are plenty of pros in bluegrass, rockabilly and country who love the sound and durability of a good ply bass. If the imported basses you mention are well made with good seasoned wood, sound good and play well, there's no reason that they wouldn't be accepted by a pro player.