DVD quality is based on using the correct bitrate (among other parameters) for the material being encoded. Material with little or no movement can be encoded at very low bitrates with no loss of quality. Material with fast motion needs a higher bitrate.
If your material is one hour or less (1:50 if using dual layer DVD) then you can encode at 8.5MbsConstant Bit Rate (CBR) for video and using PCM (.WAV) audio at 1.5Mbs. That gets you the highest bitrate you can have (if using AC-3 or MP2 audio, or have material less than an hour long you can push the video past 9Mbs CBR). You won't get any better looking output by going to QT first and then encoding to 8.5Mbs CBR using Sorenson.
The only issue of quality really comes up when using Variable Bit Rate (VBR). You need to use VBR if your material won't fit on your DVD at an appropriate bit rate. The problem here is that Liquid only uses a 1-pass encoder, and for best quality VBR you need to use a mult-pass encoder (or be able to manually encode sections separately with different bit rates). In this case, you might be better off using Sorenson. I'm not convinced that you should go to QT first, as this is already a compressed format. You'd be better off using a less compressed format (DV), or even uncompressed and then encoding that using Sorenson. I fuse to DV AVI and then encode using CinemaCraft Basic which has 2-pass VBR.
As to what settings to use for VBR, the answer is it depends on the length of the material, how the audio is encoded, and any additional resources (e.g. motion menus, special features, etc.) that you'll be including. There's a very good bitrate calculator at DVD-HQ.info (Click Here). This is what I use for setting the bitrate for my DVDs.
Hope this helps.