Wyred amps and Merlin VSM-MXe speakers

Anyone using this combo? I know the Wyred amps are engineered to work well with tubed preamps (which I currently use). I tried the STI-500 integrated but noted a bit of harshness with an Eva Cassidy song that wasn't present with my Belles 150 Reference amp and Bel Canto SEP-2 (tubed) preamp. So I'm wondering if the harshness may have been coming from the pre section of the STI-500, and thinking this may be ameliorated by my tube preamp.