If you like the overall sound of the McCormack but want to make a significant upgrade, you might try the SMc Audio upgrades. I went from a 0.5 DLX to 0.5 Rev B and the improvement was very significant, across the board. You might want to talk to them about your associated equipment. Another thought, if you are using the Theta as a digital volume control, others have reported loss of dynamics and overall "thin" sound when using digital sources without an active pre-amp. Maybe try borrowing one and see if it helps (tube pres work well with McCormack amps; I am using a BAT VK3i).
Bigger Amp?
I am considering an amp upgrade and would appreciate some (friendly) advice. I am currently using a McCormack DNA-125 to power a pair of Audio Physic Spark III's and although I really like the McCormack, I am looking to add a little more warmth and presence to my system. I have heard some say that a more powerful amp will add more presence, even at low volume. I prefer to stay with SS since I can't afford to buy/maintain a good tube amp. Can any of you big SS amp owners attest to what more power will get me (if anything) at low volume?