How to go from RCA to XLR?

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

Al the reason they don't offer balanced inputs for stereo is because they are using the two separate channels combined to make the amp balanced. One channel drives the positive and the other the negative. In stereo mode they wouldn't be able to drive the pos/neg legs of the balanced signal and keep a true differential balanced config. That's the reason you probably should not use an RCA adapter to convert an amp to single ended in this type of design. A friend had an ARC amp that only had balanced ins and was warned by a tech at ARC not to use an adapter on his amp. Balanced and bridged are closely related it appears.

Rww -- yes, but they still could have provided balanced xlr inputs for each stereo channel, and then fed those inputs into a pair of differential receivers with single-ended outputs. The single-ended outputs could then have been fed into the same path that the single-ended rca inputs in the actual design are fed into. That would add the benefits of a balanced interface and cabling, without changing the internal architecture significantly (other than the addition of a few components).

they are using the two separate channels combined to make the amp balanced .... That's the reason you probably should not use an RCA adapter to convert an amp to single ended in this type of design.

Well, ok, I don't have sufficient familiarity with this type of design to comment on this.

Regards,
-- Al