Distortions caused by subsonics are audible. Though the filter can cause some vailing it is still preferable to the distortion and speaker/amplifier demands caused by subsonics. If it is important to you to be able to deactivate the filter for good recordings it may be worth while to get one that uses a switch rather than jumpers to activate the filter. However I would prefer to have a phono-pre that does everything well (Project Phono Box SE or Tube Box) than one the just has the capability to switch quickly. Even with the subsonic filter implemented on one of these phono-pres you are likely to see an improvement over anything cheaper that offers subsonic filter.
Good luck!