The CS3.6's impedance magnitude and phase plot (fig.1) reveals a very low impedance value. The loudspeaker is under 3 ohms through most of the range, dropping to a minimum of 2.3 ohms at 3.6kHz (the cursor position). The low impedance value explains the CS3.6's need for the iron-fisted Mark Levinson No.23.5 to provide control in the bass; the CS3.6 would appear to be current-hungry.from Stereophile's measurement section (review linked below).
At 86dB with a minima of 2.5ohm on the impedance curve and staying below 3 ohm between 100Hz and looks like 10KHz, I would not pair it up with the ARC VT100. I would look for an easier load than the Thiel 3.6. Normally however, I would tell you to choose the speaker you like first and work your way backwards and get the appropriate amp to drive it. So if the Thiel 3.6s are doing it for you, than you may want to sell the ARC VT100 and get yourself a nice SS amp that has some oomph to it and that doubles its power as the impedance is halved. My 2c worth.