I have to say I haven't noticed this, and I listen to Presto samples quite frequently - in fact just this afternoon I listened to maybe an hour total of piano music samples (Beethoven sonatas, Shostakovich P&F, etc.) without any obvious deterioration compared with previously.
FWIW I was listening via PC (Chrome) / Windows10 / M-Audio 24/96 soundcard / Koss Porta Pro phones - which is not how I listen to music generally - but is the setup I use e.g. for editing music files in Audacity, and is fairly revealing of sound quality.
Update: I listened to a selection of samples from that Frémaux box you were asking about - in case it was a question of repertoire - and they sounded OK to me.