Input levels to any transmitter for any digital modes ARE critical. There are guidances published in various places about how to set up the modulation levels, some give exact instructions for good results and some are more vague and don't give such good results. I ran into difficulties running PSK31 some years ago and learnt the hard way. Now I'm preparing to get going with these new modes and am studying the setting myself so I can't give off the cuff help.
There is one general pointer I can repeat - when you arrive at a suitable level back it off some 5% or even 10%.
Good luck.
