would innuos mk3 streamer benefit from network filter

Break it in for a minimum of 200hrs, let it settle.
Also, do you have it plugged into a power conditioner? What cables do you currently have it connected with (power, ethernet, digital)?

I'd look at the DAC.  DACs can be harsh. All a digital streamer can do is send more bits to the DAC.  I also don't think wires are generally harsh.  If I wire sounds harsh, it is carrying the harshness from a component.

Jerry

Not familiar with a mark3, but have a Pulse which sounded very good out of the box connected to my DAC with an Audioquest Carbon usb and using the supplied mains lead. After a week or two of adjusting to the superb sq I switched the mains lead to an Audioquest NRG z3 mains cable and the sq deteriorated. Quickly switched back to the supplied lead (uk) and sq restored. A mystery as I use  Z3s for my DAC, SACD and Luxman amp with good results. 

You can tweak the Settings for better sound :-

Go to Settings

-> Audio -> Select low latency

-> Advanced Settings

-> Compact music library database

-> Clear queue cache

Yes, the filters, cables, etc offered by Network Acoustics do a great job on the ethernet side of things. What usb cable and dac are you using. Also, ensure that your settings in Roon bypass the dsp, etc.