Mullard never made a 6SN7 equivalent. The tube you're thinking about is the ECC32 (and possibly the ECC34). Neither is a true 6SN7 substitute and both have significantly different operating specs then a 6SN7. In some gear, it can be used safely, but in others, you can harm your component. Ask the manufacturer first. IMO, any good 6SN7 will crush the ECC32 and ECC34. Good 6SN7s can get pricey, but the Mullards are very expensive too ($200-350/pr).
While I have no direct experience, customers with the Ultranerve have told me they enjoy the RCA 6SN7GT/VT-231 grey glass and the Sylvania 6SN7W in their preamps. It seems to me the Ultranerve does best with tubes that are full and warmer sounding. Feel free to email me directly if you want more opinions on 6SN7s. There are many dozen types so I know it can be overwhelming (Sylvania alone manufactured several dozen)
While I have no direct experience, customers with the Ultranerve have told me they enjoy the RCA 6SN7GT/VT-231 grey glass and the Sylvania 6SN7W in their preamps. It seems to me the Ultranerve does best with tubes that are full and warmer sounding. Feel free to email me directly if you want more opinions on 6SN7s. There are many dozen types so I know it can be overwhelming (Sylvania alone manufactured several dozen)