Oppo 105 Analog vs HDMI?

I listen entirely to classical music, and when I listen via the the Oppo 105's multi-channel analog outs (I use the main stereo LT/RT for the front channels), the sound is rather boxy/congested and has little depth. Via HDMI, the sound just blooms! The imaging is far wider, deeper, and the sound is simply more open sounding. I find this to be very puzzling since the analog section of the 105 is so highly touted. I do prefer the m-ch outs for solo instruments since there is less of a sense of depth in the first place. Any suggestions?

(Here is my system):

DALI Epicon 2 front speakers
Martin Logan Motif center speaker
Definitive Technology ProMonitor 1000 surrounds
Martin-Logan Abyss subwoofer
Sony 5400ES SACD
Oppo BDP 105 SACD/Blu-ray player
Marantz 8801 pre/pro
Parasound Halo A51 5-ch amp