I don't think a transformer based passive is going to make that much difference here given the gain and input sensitivity of the Bel Canto amp.
The high gain CIA D-200 or D-500 would be a much better candidate to match with a passive preamp. While I have experienced the benefits of transformer based passive designs, one non-transformer design recently caught my attention that I subsequently purchased and that is the Lightspeed attenuator. You can read up more on this uuit at DIYAudio.com. You can buy one pre-built by the original designer for about $350.