Just got my Subdude HD, and it didn't fit under my A2-300.. too small.
I had some extra rubber isolators from a buttkicker install. I put the isolators at the corners of the Subdude, and lined up the Subdude under the sub. Ghetto, yes.. but it seems to work well.
The rubber alone would probably do a good job of decoupling without the Subdude. I haven't tested everything fully yet, but my closet door no longer rattles during a loud LFE moment. I'm going to test with/without subdude plus rubber isolators to see if there is a big difference.