Based on the 1661C's manual, you should be able to adjust the threshold voltage of each pod. For 1.8 V logic, set it to 0.9 V (or so).
Or have you already setup the threshold? There is a threshold accuracy of (100 mV + 3%), but that should be fine.
(And for a USB logic analyzer... I kinda like the DSLogic hardware but the software is questionable. Its logic probes are much nicer than the competitors.)
(Digilent Digital Discovery is another possible USB LA choice... one IO bank can be configured with 1.2 to 3.3V logic. The hardware isn't as nice, but the GUI is quite comfortable.)