The Dac 8 worked well as a SPDIF CD outboard dac, but it's USB from the laptop was weak. I read in some review that the Ref Dac is similar. Owned the Dac 8
As far as we see in their site they are coming with a new tubed DAC:
The DAC9 digital-to-analog converter decodes most current formats with state-of-the-art vacuum tube technology. Five digital connections - USB, RCA, BNC, AES/EBU, and Toslink - provide connectivity to all of your digital sources. Quad DAC architecture provides a balanced digital signal, wide dynamic range, and decoding resolutions from red book CD up to 384 kHz and native DSD sampling rates. A pair of 6H30 vacuum tubes are the heart of the analog circuit. Both balanced and single-ended connectors are provided for output connectivity. Native-rate upsampling and selectable digital filters allow customization of the digital signal.