I have their SP-331 power amp and as far as I know SV-236 only has line level inputs. Sounds like you're asking about their external phono preamp and its design and my guess is the tube is just used for the buffering since its a hybrid design, as the name implies.
Vincent SV-236MK
Just recently, I decided to upgrade my current Integrated amplifier which is a Vincent SV-236MK Tube Hybrid Integrated Amplifier purchased in 2009 for $1995.00 with a Luxman L-550AX which was $4990.00. Surprisingly, the Vincent SV-236MK sounded much better than the Luxman. I even purchased the top of the line Marantz integrated amplifier PM-11S3
and the Vincent SV-236MK sounded much better than the Marantz. The Vincent SV-236MK sounds warm,detailed and musical and has plenty of bass. The Luxman had no bass. I would like to purchase an integrated amplifier which sounds better than my Vincent SV-236MK but I haven't found one. Any suggestions? I have Bowers & Wilkins PM1s, Vincent SV-236MK, Marantz Reference Series NA-11S1 Network Audio Player and an OPPO BDP-105 Blu-ray Disc Player.