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You could extract the monos from the DTS with an app called Tranzcode or with Foobar (using the foo_input_dts plugin as an active DSP component to convert the dts .wav to individual mono waves). Then you could work on the individual mono waves and try to modify them with various VST's, change the volume levels and things like that.

If you have a good stereo source of the album, another approach would be to use it and go through a standard upmix. The result could be far better and the process much simpler than trying to correct the surround mix.

I have done a few 4.0 to 5.1 conversions ...

First (after extracting or preparing 4 mono wav's) you have to think "how far do I want to go, and how much more can I really get out of it?" ...
1a: Join L & R Into a stereo file, then use a center channel extraction method to extract a center
1b: Do 1a above, plus Join Lf & Ls into a stereo file, Join Rf & Rs into a stereo file, and possibly join Ls+Rs into a stereo file, then use a center channel extraction or upmixing method to obtain more channel possibilities
2: Use a multitrack mixing software to do a new mix from the extracted files + any original files

Example:
Let's say you just want to add a center channel to a 4.0 mix (making a 5.0 mix)
1. Join L & R into a stereo file
2. Extract a center from the stereo file (a few different ways to do this)
3. In the multitrack mixing software, put the original channels back in their original places (Ls/Lf/Rf/Rs) and add the center, adjust the center volume appropriately, export the mix and encode to .dts, .ac3 or .mlp

Let's say you want to do as much of a "re-adjusted" mix as possible
1. Join each neighboring audio file pair into a new stereo pair
(1=L&R, 2=Ls&Lf, 3=Rs&Rf, 4=Ls&Rs ... that's 4 stereo pairs)
2. Run either a center channel extractor or an upmixing method on each pair
3. Add all of the files to the multitrack mixing software and "be careful" ... don't put things too far out of place or the phase correllation will degrade! ... However, sometimes you'll get a perfect isolation of an instrument that was placed exactly in "rear center" or on a "center side" ... Now the isolated piece can be placed wherever you want in the mix (but again, it's at the risk of degrading or destroying the phase correlation)

IMO: It's a lot of work that doesn't always work out to what you'd expect, and many times it's more work than it's worth and I'd just rather continue listening to the 4.0 mix!
[EDIT]Message edited to correct usage of "+" to the proper symbol "&"[/EDIT]

is very clever!
i have few DTS 4.0 where the basses are fadeds(without life) and of course without center!
i was without courage to redo as will be lossy over lossy but after your 'how to' to extract Center, deserve some cool editions, add the Center as you posted more few LFE.

thanks so much, very fine guide, i never saw seamless before!

cheers!

Thanks for the suggestions. I have done some work with this many years ago, but it was very labour-intense and I it seems the current methods give you a lot less to do manually and I might start again.

What multi-track mixing SW for PC could you recommend? Both free/spam and commercial. Is anyone good for doing panning?