How to connect sub to Cary SLP 98 preamp?

Cary SLP98 preamp has 2 pairs of outs. I am using one pair L+R to connect left and right Bryston monoblocks, these apms are powering L/R speakers (I have 2 channel system only).
Now, to get a better bass, I hooked my Totem Storm subwoofer to second output on Cary preamp, using Y-splitter at preamp and then running just one RCA sub cable to the sub. The subwoofer has low level RCA L/R In and also RCA L/R Out. I connected a single cable fron preamp to left In. I think there is no difference if it is connected to L or R.
Now about the problem: Cary preamp has left volume, right volume and a Master volume. Master volume controls both channels of course and left/right volumes control L/R channels only (as it is supposed to do).
BUT, with my sub connected as described, left volume knob controls BOTH channels and right volume knob controls BOTH channels as well ?????
I thought this was a right connection for the sub but now am puzzled. Any idea what is wrong with this hookup? What is better way to hook it up (I do not want to use speaker level connections).
Thanks for the help.