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.
I don't know why the addition of the Y adapter causes the L-R controls to operate both channels but if you use a stereo pair out from the Cary to the subs L-R inputs (which is why they are there) as opposed to the Y, I think the problem will go away.
Why don't you want to use the speaker level connections? It's the best way to do it hands down. It doesn't change the final impedance the amp is handling, it doesn't draw any power away from your main speakers, and it keeps the subwoofer in better synch with your main speakers because it will be seeing the EXACT same signal that your speakers see.

Just run one pair of speaker cables from the amp to the speakers, then another pair of speaker cables from the amp to the subwoofer. Don't daisy-chain the speakers to the sub's speaker level output because THAT will be detrimental to the sound.
why don't you just run RCA's from the Cary's second output to the corresponding RCA inputs on the Sub? That's what they are there for.
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Thanks guys for the help.
I tried to use two RCA cables and the problem was gone. So there certainly was something wrong with that Y-splitter connection.
Now I have to decide which connection option to use, either low level with 2 RCAs or speaker level, but I am leaning towards the speaker level.
It sounds like you have MERGED the second left and right into a single RCA channel instead of splitting them. That was not a good thing to do. Just connect the right channel to the sub, as we commonly do.
I had the same question and it was answered. Now I am wondering if anyone has an opinion on cable quality? Right now, I am running old school Audioquest interconnects, mixed with higher quality homemade speaker cables and now junk RCA's to the sub. Is the system only as strong as my weakest link? I have my volume turned to nothing almost and this B&W ASW600 is overpowering the B&W 603's? Any recommendations? Thanks- Johnny