How to go from RCA to XLR?

Al, if truly balanced is achieved best with mirror imaged circuits, what better way is there than taking a stereo amp and letting one channel drive the positive signal and the other drive the negative? I know this seems like a simplistic approach but it should work maybe at the cost of low impedence drive.

Rww -- I'm not certain I understand your question, but I think you are asking why shouldn't all monoblock amps, at least the better ones that have balanced topologies, be designed as bridged amps.

I'm probably not the best person to give you a comprehensive answer on that, because although I am an experienced EE my professional background is primarily digital, not analog, and is unrelated to audio. But the basic point to bridging is, as I'm sure you realize, to increase output power capability. But that comes at the cost of the ability to drive low impedances, as you alluded to, and I'd imagine at the cost of a number of other conceivable distortion mechanisms. Off the top of my head that would include difficulty matching the two amplifier sections precisely in terms of many different parameters (gain, linearity, phase and frequency response), delay offsets between the inverted-polarity path and the non-inverted polarity path, etc.

And whatever the reasons may be, I think the conventional anecdotal wisdom is that more often than not bridged stereo amps often just don't sound as good as when those same amps are used in normal stereo mode.

Let me know if I misunderstood your question.

Regards,
-- Al

Al, thanks again for your answer and I see your point. But a stereo amp should also be closely matched as well. I am certainly no expert which is obvious. But an example of such an animal is MBL whom offers some of their most costly designs as either stereo single or a mono pair that simply uses the XLR input to make them balanced mono only. Such a design cannot be used in single ended mode as a mono amp. It could be used as a single channel amp in single ended mode but only with one channel which would be half as much power or less.
http://www.mbl-germany.de/Reference_html/9007.html

So both output amplifier sections would still be driven, and therefore the result would be no reduction in output power capability, just a 6db reduction in gain as I and Atmasphere indicated.

This is correct. The problem of using RCA to XLR (apart from figuring out what is + and -) is that you are grounding one side of a balanced circuit. It will make your setup much more susceptible to ground loops and ruin its immunity to noise. Balanced means just what it says - everything is balanced with respect to ground and noise of ground loops (from power supply leakage, different ground points etc.) become irrelevant as they affect both line level signals equally.