RCA vs XLR which would say is the BEST?????

Here is the system:
1. Merlin TSM-M (right/center/left)
2. Axiom QS 8 rear channels (right/left)
3. (2) Earthquake Supernovas MKIV
4. Satori Shotgun Bi-Wire Speaker Cable (right/center/left)
5. Arcam DV 27 (no xlr connection only coax/optical/analog)
6. Bryston SP-1 Pre/Pro (coming with XLR connections)
7. Bryston 9BST amp (coming with XLR connections)

Now I need to connect the SP-1 to the 9BST so do I use RCAs or XLRs? I can do the in either .5 or 1 meter. I have enough RCAs to do the job now and I would have to buy the XLRs. I only want to do this once (right how many times have I said that in this hobby) so your advice will be welcomed. Which should it be?????
Lots of variables. Depends on the electronics. There will be a sonic difference just in using different cables. In my experience comparing same brands of cables (1m lengths) RCA is just fine, no benefit from XLR balanced cables.
If a component is truly balanced do it, if not it doesn't matter much and in fact the single ended(RCA) may sound better. I don't hear many people using truly balanced components claiming they sound better when running single ended cables. Having said that I prefer balanced components with balanced cables.
I second Tim's post.
Bryston is a great amp, but it is not truly balanced, and you are not using long lenghths so RCA's are just fine.
I recently set up a sound system for an event at our church. I got everything up and running, then finished running some cable. I ran a cable with single ended 1/4" plugs about 20' across the floor. Soon after that I noticed a hum. It did not take long to determine that this cable was picking up an induced hum from an extention cord that was running next to it. Separating the audio cable from the extension cord solved the problem. However, I tried running a balanced cable next to the extension cord, with a 1/4"/balanced adapter plug at the source end. This also solved the problem.

The point is that there is some benefit from running balanced in that it can reduce background noise. Noise is the curse of many audio systems, and I think one reason balanced cables sometimes sound better is due to reduced noise. But you can also do a lot of good by not jumbling all the cables, power and audio, in a pile behind the equipment. Separate the cables, or at least make sure they do not run parallel to each other.

Happy listening.
Abolutely correct balanced can reduce hum and is of course used by many musicians for stonger transfer of signal with long runs.However the poster will be running .5 to 1 meter.
A lot of the NOT truly balanced preamps still generate twice the gain at the XLR outputs. This may reduce the useful range of your volume control. Check the specs to see if the output of the Bryston doubles between the RCA and XLR outs. Then let your ears be your guide.

In my experience XLR's always sound as good as, and usually better than, RCA's. It might be imaginary but they give the sound a more airy, floaty quality.
A lot depends on what gear you want. There is a lot of great gear out there that does not come with XLR connections. If you can use XLR's and it is truely balanced then go for it. Definitely do it for very long runs. I agree that for short runs, you probably won't notice much of anything if any at all.
I have heard some positive differences using balanced connections for 1m runs vs. same kind of cable single-ended. I don't think anyone can really know this answer short of you trying it out for yourself, despite your wanting to "...only do this once". To me, if you want to know for sure, you'll have to bite the bullet and experiment (just make sure to match volume levels to compensate for balanced's hotter output). At least buying cables on Audiogon if you're so inclined, you probably won't have to worry about taking a hit in the wallet when you go to sell the loser.
Having read all the posts above and found nothing to contend with, I would consider Zaike's post the most reasonable., simply because there is no objective "BEST" . Your ears will have to decide....just my 2cents ..Cheers,

Can this be done, shouldn't be done, or you can but it will sound like #%$@!!!??

Seeing that I have five speakers (Left/center/right/ and two rears) and only three single xlrs (at this time) can I run the left/center/right in balance mode and the two rears with rcas in an unbalanced mode? Or is this one of those either or things? I truly want to do this right and it looks that the xlr way is the one. What do you think?
Well a potential problem doing this is XLR's do indeed generally produce a 3db gain over RCA's , so you may find yourself in a situation where the fronts over power the rears. Of course adjustments can be made on your preamp, but that can be a little tricky. I use an all balanced system so I know the benefit of XLR cables , but in your case, my guess is they are not necessary. However that being said, you will always wonder what your fronts sound like with XLR's , so see if you can borrow some. In some systems XLR's are provided, but only as a marketing ploy.
Many Rotel owners, for example, prefer there amps hooked up single ended.
In your current set up, unless your interconnects are above 2m length or more, XLR will absolutely do nothing good to your system. In fact, RCA should be a good choice.

However if your system is fully balanced i.e your preamp, cd player, amp are truly balanced design then XLR would be the only choice. The bryston combo you have does provide balance output but they are not fully balanced design nor your Arcam player.
I have a pre-amp that is truly balanced (Bat vk5i) and an amp that has XLR imput but not truly balanced (Bel Canto 200.2). I'm currently running RCA between the two (with adapter on pre-amp). I'm curious whether my system will benefit from running XLR cable instead.
The configuration of the gear can in fact be a mix of up to three possibilities between the various components: fully balanced, balanced I/O but not internally, or single-ended where the XLR jacks utilize only the hot portion of the signal. Either of the first two configurations might be worth experimenting with balanced connection, though even in the first case and especially in the second, results might not be clear-cut in favor of balanced. There are really four auditioning outcomes possible if it's a controlled test (meaning done with the same kind of cable both RCA and XLR, and at matched volumes):

1) You hear no difference.

2) You maybe hear a slight difference that you're not sure is an improvement, but anyway is too slight and elusive to make switching worth the trouble and expense.

3) It makes the sound worse (yes, it's a real possibility in some situations).

4) It makes the sound definitely better and is deemed worth the trouble and expense of switching.

Even in the case of gear that only provides XLR jacks which are effectively single-ended, there could still be a difference in the sound due to nothing more exotic than the fact that a different type of jack is being used.

So I beg to differ with the absolutists going either way. A lot of this question also depends on the hearing accuity of the listener, the resolution of their system, what they define as being a worthwhile improvement, what their system and listening priorities are, what the budget is, and what kind of cables they're using now. And all bets are off if the balanced cables they're considering are of a different kind than the RCA ones they're presently using. All of the preceding is in addition to the question of what the configurations of the partnering gear's I/O capabilities may be, which in itself also contains both the theoretical and the actual aspects of performance, which in turn are also affected by the environment the gear is used in. And then there's the question of possible differences in the handling of the ground connections, which could have effects on the noise floor...In other words, there are just too many variables to start laying down absolute laws about outcomes regarding this whole issue.

So as I said before, the only sure-fire way to know what's up for you in your system is to try it out and listen. Hammerfilms (great username, BTW!), since you already have some balanced IC's on hand, what you need to do is simply run the experiment yourself using just the stereo mains and leaving the other channels disconnected (to eliminate potential additional ground-plane noise). Obviously, the most diagnostic test would be to use the same kind of cables for both RCA and XLR, and try your best to compensate for any level diffences using the volume control prior to serious auditioning. Time go beyond asking the forum to stand in for your own ears! :-) Let us know your results when you do...
How does one know if a component has truly balanced outputs (do you refer to the output impedence?). I have an MBL CDP2 which has physically both XLR and RCA outputs.
Dtanclim, short of opening up the unit and looking at the circuit (not something that will suffice in all cases or for all people), I would simply call up your dealer or the distributor and ask them - that is, if the info isn't already on the maker's website.
Dtanclim has asked a question that I should of asked. How does one tell? To all that have replied and read thank you for your time and trouble in this matter.