While many focal reducers and flatteners are threaded to accept filters, or to connect to M48 accessories, generally, putting a filter in front of the reducer or flattener optics is not recommended for two reasons. Firstly, it tends to cause optical aberrations, resulting in a softer image overall, and distorted star shapes, especially at the edges. This difference can be seen if you have good guiding, good conditions, and good optics. Secondly, it’s advisable to put the filter closer to the camera, because this tends to reduce reflections and halo artefacts around bright stars caused by reflections off the rear reducer lens. Therefore you should put any spacers in front of the filter holder or wheel, if possible. Here’s a more in-depth knowledgebase article about getting the correct spacing with your reducer / flattener.