[steveh]

Sound always arrives at the TiVo from Virgin in a lossy format: Dolby Digital / AC3 for most of the HD channels and MPEG Layer 2 audio for SD channels and some HD channels (e.g. BBC News and ITV). The MPEG Layer 2 audio standard dates back to the end of the eighties and is easily outperformed by Dolby's AC3 codec.

If you have the TiVo set to deliver PCM to your home cinema system then all you are doing is having the AC3 conversion done in the TiVo rather than in your home cinema system for those channels in Dolby Digital. As Spiderplant said, the TiVo conversion will be inferior to what is capable by the dedicated chips in a more expensive home cinema system.

The TiVo does seem to do the conversion at a higher volume level than you get passing Dolby Digital through to a home cinema system, so it may be that perceptual difference ("richness") that you are hearing. The Dolby format includes extra data on how to optimise downmixing multi-channel sound which not every device will support.

Should note that the TV companies also often process the sound differently between the SD channels and HD channels with greater compression of the SD sound levels - i.e. fewer differences between the loud and quiet bits. Dolby define a standard level for speech in a soundtrack which is considerably lower than traditionally used in TV but allows for loud effects and music in movies. That's another reason why movies on a channel in Dolby Digital sound quieter. Just turn up the volume to appreciate the full dynamic range!