If you just want something that makes the HD800 sound a little different than it does with the Mainline you could consider the Crackatwoa. If you just want something with big tubes as you have suggested to me before, a Kaiju will sound more different from a Mainline than the Neothoriator will. Kaiju will be a little more bass/lower midrange-rich (as will the Crackatwoa), Neo a little more neutral like the Mainline.