How to go from RCA to XLR?

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.

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.

Rww -- I took a look at the MBL 9007 via the link you provided. Yes, it pretty obviously is designed to work in a bridged configuration when used as a mono amp, since it can also be used as a stereo amp. Just goes to prove the old saw about the design approach that is chosen being less important than how well the particular approach is implemented. Not sure why they didn't provide balanced inputs for stereo use, though, unless they just felt that from a marketing standpoint those who would buy just one amp and use it in stereo mode would be less likely to be driving it with a balanced preamp than those buying two amps for monoblock use.

Such a design cannot be used in single ended mode as a mono amp.

Not by going in through the single-ended inputs, but I would think you could do just what we have been discussing, namely going in through the xlr connector with an adapter that grounds one of the two polarities.

As Shadorne states, that would of course sacrifice the noise and ground loop-related advantages that the true balanced interface would provide. But I think it is important for the OP to keep in mind that the results he can expect in those respects would not differ dramatically from those that a comparable fully single-ended (unbalanced) system would provide.

Regards,
-- Al