I looked at Combined, after having an insurance broker check with all the other insurance companies for Income Protection Insurance . As soon as MX was mentioned as a sporting interest they all baulked. The only one I found (apart from Combined) was AIG (now called AIA)
The policies are very different, you'll need to sort through the details and decide for yourself, but I went with AIG for a number of my own reasons.
As an aside, I found it ridiculous that every single insurance company wouldn't have a problem insuring me if I rode a 1200cc road bike through peak hour traffic every working day of my life, but if I ride vintage MX once a month they weren't interested