I would try a different source if I were in your shoes.I haven't spent any time with your DAC,but the source could very well be the root of the problem. I'll suggest a Electrocompaniet ECD-1. I've been auditioning a modified version of this Dac(thanks to a friend) using a passive volume pot. No tubes in the system at all.The sound is VERY full bodied and slightly warm with no hint of detail lost.It seems to do this without getting strident or bright.The soundstage is stable and wraps around you ...if your speakers image properly. This is a very well balanced dac..never calling attention to just one part of the music.
Good luck with your hunt!
Good luck with your hunt!