I also have this issue. I was with TalkTalk before, and never had a problem with their box!! I was watching TV with the sound around 22-24 now it has to be on at least 30 on my Pioneer amp. If I then use my Sony Blueray player to watch catch up using the channel 5 app, I have to turn the sound back down to 22, which proves the issue is with the v6 box, not sure why Virgin is saying it is with certain TV's!! as what I have just said proves my TV (Which is a Celcus) does not have an issue. Can't Virgin give us some idea on progress with this issue? As it has been going on for over a year now!!!
Having the same problem with both of the V6 boxes we have, but I think I've found a solution. Both of the TVs are newer LG smart TVs so this might not work for everybody, but enabling Auto Volume on the TV itself fixed it (settings button on LG remote > go down to All Settings > Audio > Volume Mode > Auto Volume).
After switching that on, comfortable listening volume is now at 17-18 rather than 40 (and it's consistent across other apps on the TVs like YouTube/Netflix/Amazon Prime Video).
I think it is clear there is an issue here, weather Virgin admit it or not, I still have this issue. Because of this issue my AV volume setting is between 30-34 for TV, but today I used my Song Blu-Ray player to watch catch up on Channel 5 as there are no adverts, but the first thing I have to do is turn the volume down to 22 which proves to my ears that there is an issue with V6. It has been dragging on a very long time now. What about people who work for Virgin, they must have the kit at home, are none of them not experiencing this issue? I find that hard to believe.
The original post for this topic was March 2017, we are now in August 2018. The clear message is Virgin is not bothered about this!!! It would be better for anyone new wishing to install Virgin V6 to let the engineer hear the sound level of your current kit before he installs the V6 then compare, then maybe they will agree there is an issue and inform Virgin?