Views:

On both of these, looking at the front, the input is on the right; so explaining the tubes from right to left will make more sense.

333XL INFINIUM - signal flow, viewed from front, right to left:
Clean channel - 1st tube, 3rd tube, inverter tube in the rear with the power tubes, then the power tubes.
Crunch and Lead channels - 1st tube, 2nd tube, 3rd tube, inverter tube in the rear with the power tubes, then the power tubes. 

TRIREC INFINIUM - signal flow, viewed from front, right to left:
Clean channel (channel 1) - 1st tube, 2nd tube, 4th tube, 5th tube (the inverter), then the power tubes.
Crunch and Lead channels (channels 2 and 3) - 1st tube, 2nd tube, 3rd tube, 4th tube, 5th tube (the inverter), then the power tubes.

In a nutshell, both of these amps flow right to left, sequentially.  On both, Clean just skips over the middle tube, which is where Crunch and Lead pick up most of their distortion.