Joe, lots of people soften their Class D with tube preamps, so that is doable, but I don't see how you can use an amp as a phono preamp. Initially I thought you were going to use the preamp section of the Willeston into the class d, but it looks like there is no pre-out only a pre-in, so that won't work.
The cheapest tube preamp I would recommend is the Quicksilver.