Marian Trace Alpha sound card

I'm just guessing here.
Comparing quickly the features of Marc 4 and Trace Alpha my first (and only :o) thought was that it has something to do with the mixer function of the Trace. Otherwise, I don't think I've ever heard that changing soundcard messes up the mix. Slightly changed EQ-balance or sound otherwise, yes, but a messed up mix....never heard.

"DSP Mixer

The digital mixer - completely integrated in the TRACE hardware - operates with quick and flexible DSP functions allowing latency-free mixing of all signals to several stereo sums as well as the latency-free monitoring and routing of all inputs to all outputs. Thus perfect monitor mixes can be set up quickly and played back on any output. The mixer can be configured optically - all inputs/outputs and many mixer sections can be either shown or hidden. All audio-, routing- and layout settings are easy to save and load within session files (total recall).
The Mixer offers 20 channels (8 x internal Playback, 8 x TDM Input, 4 x physical Input) and - depending from the used samplerate - up to four individual stereo sums.
Output routing control: Independent from the mixer the systems provides six physical stereo outputs - two physical stereo outputs plus four stereo TDM busses. All outputs can be driven with the mixer stereo sums, 16 software playback channels (see below) or both physical stereo inputs. Multiple routings are of course possible."

You should be looking at a proper audio interface, not a computer sound cards. There are to many threads here ( and all DAW forums)  with people banging there heads against the wall because they are trying to run a DAW on a consumer gaming card. 
And you don't really want any DSP mixer between your DAW and your monitors. It will give you a false impression of your mix. 
All audio processing must be within the DAW and then the export will sound identical every time. 

Hi Goddard - Very many thanks. I did check that Windows system sounds and PC onboard audio were disabled but didn't check sample rate etc in the project from the old card to the new - could well be the issue and I will check this out - again, very many thanks. (Absolutely agree with your comments about Marian interfaces etc - after trying a range of cards in my (very low!) price range, I felt and still feel the Marian Marc 4 Midi was by far the best - a lovely warm sound ideal for my music - the only reason I felt I had to upgrade was that it is fine for XP but with the constant changes in Windows Operating Systems, Marian will eventually stop supporting it - and that will be a very sad day!).