Troubleshoot, piece by piece. I'm going to go from left to right, for simplicity.
1) Set the input impedance high. Dull passives are usually due to a low input impedance - you're going to want 2M or 10M here.
2) Phantom power, ground lift, phase flip and speaker soak off. The speaker soak has a low-pass filter built-in, if my memory serves me correctly.
3) Gain up 'til you get the OL light to blink on really hard hits. Some preamps sound dull when you've got the gain anywhere below "about to hit the rails." I don't remember the TD-1 exhibiting this behaviour, but you never know.
4) Skip the EQ for now - just turn it all off from the switch on the far right.
5) Try the SS input circuitry to start - the tube might be dying, as unlikely as that is, and that would have a detrimental effect on the tone.
6) You've got the last switches on the front panel set to Instrument and pad out, of course.
7) Back panel, use the balanced line out. The XFMR Out puts out mic-level signals, if I remember correctly. Also, make sure you're not plugged out of the Reamp Outs. Hey, you never know.
Good luck!