Yes, it is possible to keep the TiVo as a second box and stream content from it to the V6, but you would have to pay the fee for having an additional box and would most likely be tied in to keeping this as part of the package for 12 months.
I called Virgin yesterday. They were extremely unhelpful! The person I spoke to told me that every option I explored would be impossible, and even having fewer boxes than I have now would cost me £30 a month MORE!
They're hopeless, and I'll now be exploring ways to leave Virgin and move back to Sky.