in my experience, the superiorty of optical interconnect over coax becomes more apparent as the quality of your transport/dac increases. in my setup, i have optical, "six-9's" coax and toslink connected simultaneously between my tranport (accuphase dp-90) and dac/digital preamp (accuphase dc-330). with my remote i can switch among all 3 links. i've done this for myself and and numerous friends whose opinions i respect. in every case, on every cd tried, the optical has come out on top. it is more open, has greater clarity and detail and is ultimately more musical than the other 2 alternatives. i haven't listened to the theta/krell for awhile but recall preferring the optical on this combo, too. if i were you, i'd concentrate on upgrading both your transport and dac before spending the $400 or more a piece of really good optical cable will run you.