how to best a/b cables

Ok, I admit it: I have serious doubts about the claims of those who believe in cables. But I am willing to take their challenge - let my ears decide. So I went to a local dealer (who is probably reading this thread - Hi!!) and got some Transparent speaker cables and interconnects to audition. My setup: Linn LP12 turntable, Linn Linto phono stage, Linn Ikemi CDP, Plinius 8200 integrated, B&W N804 speakers and 10 year old Audioquest cables & inters (I believe they are the "Ruby" models.)

My question: my Linn Linto has dual outputs. Can I connect my old cables into one output and the Transparents into the other and then run them into two different inputs in the line stage pre and thereby get a good way to a/b the cables? My dealer says it is not a good way to get an a/b comparison because the two sets of cables will affect eachother and it will muddy the water. Does this make sense? If I hook one into the tape1 input and the other into the tape2 input and if I'm switched to tape1, then tape2 is an open circuit and should be completely inert and vice versa. Any comments?

I think your dealer is right. I cannot give you the technical reasons for it, because I'm not trained that way, but my experience and the rules for this sort of experiment point in the direction, that the candidates or devices under test should meet exactly identical conditions. In your case this would mean swapping cables and plugging them into identical in- and outputs.
Try it your way. The switch the cables from one output to the other and one input to the other. See if you think there's a difference that travels with the cable when you make the changes. Then do it your dealer's way. If the differences between inexpensive 10 year old cables and expensive new ones are so slight as to be masked by the fact that another cable is plugged in to the line stage, I would wonder why you would want to spend the money.
If both the Linto and the line stage have selector switches, then it is an open circuit. If the Linto does not, and both output terminals are always hot, then the presence of the second set of cables presents a slight load at the Linto output, which is different for each cable set and can result in small differences in the total Linto output. You can test whether any of it is audible by listening to one pair of cables w/wo the other set plugged in.
As to the minor difference in pathway between two different sets of input posts or output posts which are switched on the inside of the box, I think you'd have to be very golden-eared to hear the difference (assuming good build quality), but the strict rules of A/B'ing say that you should use the same terminals. I suspect the cable differences will greatly exceed the minor differences from this source. After all, A/B switching boxes are built this way, though admittedly they should be built carefully.
I think your approach will work just fine--much better than switching cables in and out, which introduces a time delay that will be far more confounding than any minute increase in load from having a second set of cables attached.

As long as you're going to this trouble, get a friend to help you do the listening blind. He can switch back and forth between A and B (at your instruction, so you can listen to them for as long as you like). Then let him flip a coin to decide which is "X," and see if you can tell which it is. If you can't score 80% on a reasonable number of trials this way, you can be reasonably certain that the two cables are sonically indistinguishable.
Also be carefull as to 'when' you flip the switch. If a loud passage is coming up just as you switch, you might belive you heard a tonal shift. Its actually very hard to switch between small sements of music, but if you wait for a whole track you will have forgotten what you just heard.

One weay or another this will be educational for you. Your dealer sounds 'closed minded':).

I find quick switches back and forth do not work for me. I like to listen to a long passage of music, then switch and replay with the other cables. It takes many switches back and forth to really get a good handle on the all the differences between the 2 cables. I miss things when it is a quick switch back and forth. We listen long, so you should compare long. The negative fatigue factor won't become apparent with a quick switch. Neither will the good musical toe-tapping factor.
I agree with Sugarbrie and will go one further to say that quick back-and-forth switching actually obscures sonic differences between cables (or components). But try it both ways and see for yourself.
Sugarbie's and Drubin's points are well taken and I tend to agree. But if the claim is that the difference will be "jaw dropping", shouldn't that be apparent using the quick a/b method?
I've spent alot of time testing cables this past year. I found quick A/B to be less successful than three to five day tests. Play a set of disks over a three day period. Spend time listening to the detail and also spend time simply enjoying the music. Now switch the cables and play the same disks over the next three days.
I found that certain cables were extremely enjoyable and brought out more detail. Others left me less involved and less impressed.
I must agree with the above posts though. If you can't hear it right off, than why change?
People who study hearing for a living use quick switching because they've found that humans do not remember subtle differences for very long (like measured in fractions of seconds). But there are lots of audiophiles who believe otherwise. Nonetheless, you can do ABX testing any way you want, either quick switching or listening to whole passages. Just don't expect it to be easy!
Generally I can hear differences in equipment in the first split second of being exposed to the sound and Bomarc is right, the ear adapts to the new sound very quickly and the brain soon regards it as normal. It therefore makes good sense to pick out certain passages of recordings one knows well and concentrate on how different DUTs will play them. Only then, I find, you will get a grasp on what the differences really are. These differences, when heard on these distinctive passages of music are then far easier to memorise, especially if you have found a language for yourself to describe them. After that, at least to me, it makes sense to listen to the devices under test for longer stretches, making notes all along, to base your judgement on as many parameters as possible.
If a difference is so subtle that quick A-B switching means you can't distinguish the difference then I certainly wouldn't pay money for such a slight difference. Beware the oil of the snake.
A very reasonable collection of posts for this topic - Gboren's imploring request seems to have worked! But to all of the above, I would add: Don't forget to allow the new cables some break-in time of being in the system continuously for several days before passing judgement. (For the record, I would definitly try the dual-outputs and switch method first, despite what the dealer says.)
I think your dealer, like so many dealers, is full of hot gas. He is trying to influence your decision and make a sale by impressing you with this bit of hooey about two sets of cables between different outputs and inputs affecting each other. I find this intimidating techno-babble reprehensible. Equally upsetting is all the nonsense about elaborate testing set-ups. You have an absolutely ideal set-up for testing two pairs of IC's. Asking a friend to randomly switch between them is good advice, as is letting them both settle in the system for as many hours as possible. Then switch between them whenever you feel you have a grasp of what one set sounds like. It can be minutes or hours or songs or movements or phrases. If the music sounds more like music, that's the better cable. Forget highs and lows and soundstage and all that junk. Which one makes a violin sound like a violin? Have fun, and don't take this too seriously.
I wasn't going to comment on this, but the misinformation in some of the above has caused me to reconsider. You have two pair of phono outputs in parallel to each other. You take two pair of cables, only one at a time terminated into the preamp depending upon which is selected. The inductance/capacitance of the unterminated cable in parallel to the signal-carrying cable can definitely influence the behavior of the "working" cable. I've heard this myself so I know that it can happen.
The only way to determine if this is a consideration in your own application is obviously to experiment; it may or may not be an issue.
I agree with Bob_bundus. I have experienced it with my set-up, even measured the difference. Not great, but nonthless present enough to change freq balance and thus sound. Safer to connect and listen individually and be certain.
I have tried a method similar to what you propose and found that it only "worked" if the cables were significantly different, like, say, copper vs silver. When dealing with cables that are fairly close in sonic characteristics, I found your proposed method can sometimes be confusing and frustrating. I prefer the approach advocated by Sugarbrie above. This is supposed to be for fun and enjoyment (I have to wonder sometimes) so I would avoid any approach that is going to put any pressure on yourself. I also wholehearedly agree that if you find yourself singing along or tapping your feet this is much better than sitting there clenching your teeth or wondering what differences you are supposed to hear, so if one cable makes you more relaxed than the other, this is usually the "keeper" (this has worked for me for the past 20 or so years).