I ordered an optical cable yesterday so I could compare the optical and analogue outputs. My stereo amp does not have a DAC so I used a low budget ( £200 ) 2.1 AV unit I have connected to my TV.
I am pleased to say there is a noticable difference in clarity between the 2 outputs, the "soft" sound I experienced with the analogue output is not present with optical. So I think the way forward is to get a decent DAC and power Amp combination to drive my Mission 753 Freedoms.