One challenge you have with the ETX is that you don't have a lot of light to work with so you gotta be a little careful with filters. I prefer to use sky glow filters that are designed for color imaging. These remove narrow bands of light to leave as much behind as possible to keep from whacking the color ballance. The two that I've used are the Orion Imaging Skyglow Filter and the Hutech IDAS LPS2. I've also used the Orion Broadband and Ultrablock visual sky glow filters, but they remove too much light for my taste (though the Ultrablock is an interesting filter to use with planetary nebula). The only visual filter than I've been happy with is the Baader Semi-Apo filter. For visual use with a reflector you don't need the V-block part of this filter, so the Baader Moon & Skyglow filter should work as well.
A word of caution; the benefit of filters for visual use can be subtle, so you might want to see if you can look through one before buying. If I were to pick one I'd probably go with one of the Baader filters as a ballance of cost and performance.