Poor quality audio from analogue line out

I’ve got Kodi running on a Linux box connected to my TV via HDMI – I’ve configured Pulse Audio to allow simultaneous output to both HDMI and the analogue out on the motherboard which works brilliantly (both perfectly in sync which is great) BUT the output from the analogue connector sounds very poor – it’s hard to describe but it’s like the resolution is too low so sounds scratchy and squelchy (lots of hi frequency in the sound).

I can’t see anything in Pulse Audio that would drive the output resolution and I’m not running a desktop environment (Kodi runs under Openbox).

Is there a setting somewhere that I can set/check?