Threshold's own fet10 combo will work very nicely. Audio Research and Sonic Frontiers (though I think with the S300 many of the benefits might be lost)) are reputed to be good tube matches. FWIW, Audio Research is also reputed to work well with Maggies. If cost is an issue, the Forte pre's might be worthy of consideration. I'm guessing that with the e series, if you want to use a tube pre, it might be prudent to stick with one of the few DC coupled tube pre's such as Atmasphere or Sonic Frontiers.