Dual Differential / Balanced?

@1 - I've never said that it doesn't.  But it will remove cable noise by means of either balanced input in single ended amp or fully balanced amp.
 @2 - again, I did not say it removes even harmonics from the signal. It removes even harmonics produced by the amp at any stage by means of cancellation AT THE OUTPUT.  These even harmonics often make amplifier sounding warm and euphonic.  Yes, the third one is not bad, but higher odd harmonics are quite nasty.  Removing only even harmonics while leaving odd harmonic intact will lower total THD but also will make sound colder, brighter.
@3 - Output impedance will double since there will be two output stages - similar situation to bridged amplifier.  Complexity is much higher no mater how simple the circuit is. If signal goes thru one gain stage only in single ended design, it will go thru two stages in the balanced one  (not to mention two output stages).  You might not see a problem with that, I do.
I won't even comment on the issue of instability because you can find it in any FAQ/primer on fully balanced design, but it appears that you question my notion that connection with XLR cables is not always the best.  Experience of many people on this forum was that RCA connection sounded better than XLR.  I'm sure that Al (our technical guru) would agree with me - that balanced output, because of additional circuitry might not always be the best.  I will leave it at that.

kijanki

 " ... I'm sure that Al (our technical guru) would agree with me - that balanced output, because of additional circuitry might not always be the best.  I will leave it at that."

I'd be surprised if anyone here would disagree with you on that, kijanki. It's especially true on devices that use cheap op-amps to process the balanced signal into a single-ended one.

Ralph (atmasphere) responded to your other confusions, so I'll leave it at that.

Nyhifihead
from what I’ve read it only matters if both the preamp and power amp are both truly balanced

That is a belief that is shared by many audiophiles, but it is a misconception IMO. A **well designed** balanced interface may be advantageous regardless of whether or not the internal signal paths of the connected components are balanced or not. And a **well designed** balanced internal signal path within a component may be advantageous regardless of whether or not the internal signal paths of other components in the chain are balanced.

The potential advantages of a well designed balanced interface include reduced sensitivity to cable differences, reduced likelihood of ground loop issues, as well as reduced susceptibility to noise pickup. Of course, as Kijanki mentioned it is not uncommon (especially in modestly priced equipment) for balanced interfaces to perform worse rather than better than single-ended interfaces that may be provided on the same components.

Cleeds
I’m pretty sure that the Integra Research RDC 7.1 is not "fully balanced."

+1. I say that in part because searching the 156 page manual for the terms "differential" and "fully balanced" yields no hits.

Nyhifihead
I was under the impression keeping the signal balanced all the way through with true dual differential output/inputs on the pre and power amp would be the best course of action?

As is usually the case in audio, how good the results are depends less on the design approach that is chosen, and more on how well the chosen approach is implemented. As a general rule of thumb I would expect that monoblock amps, dual mono amps, and fully balanced amps (which mean different things, as Cleeds indicated) would tend to have the greatest likelihood of being preferable choices at price points that are significantly higher than the price range you referred to. Especially given that you need fairly large amounts of power.

Kijanki
...connection with XLR cables is not always the best. Experience of many people on this forum was that RCA connection sounded better than XLR. I’m sure that Al (our technical guru) would agree with me - that balanced output, because of additional circuitry might not always be the best.

Thanks, Kijanki. Per my first paragraph above, yes, I of course agree. It depends on the specific implementation. I’m pretty sure Ralph (Atmasphere) would agree as well. As he said at the end of his most recent post above:

...the McCormick and Pass amplifiers are quite good and accept a balanced input correctly (many high end audio amps do not, likely because the designers don’t know that there is a standard for balanced line operation, defined by AES file 48).

Regarding balanced outputs having twice the impedance of unbalanced outputs, in the majority of cases components providing both balanced and unbalanced outputs utilize similar output circuits for each of the three signals that are involved (two balanced, one unbalanced), and in fact may often drive the single-ended output with one of the two signals in the balanced signal pair. The usual convention is that the balanced output impedance is defined based on the sum of the output impedances of the two signals in the balanced signal pair, so the output impedance number for the balanced output is often twice the output impedance number for the unbalanced output simply by virtue of that fact. Therefore, each of the two signals in the balanced signal pair will often have the same output impedance as the unbalanced output signal that may be provided by the same component, and the statements about output impedance by Ralph and Kijanki are in a sense both right.

My bottom line suggestion to the OP: Don’t choose based on topology. Although I’ll also say that in your stated price range, and especially given your power requirement, my expectation would be that more often than not you would end up with best results choosing a stereo amp (on a single chassis) that has an unbalanced internal signal path. And if it provides both unbalanced and balanced inputs, try both.

Also, for the OP’s info, the advantages and disadvantages of balanced differential design (that being one form of balanced design) are nicely explained in this paper at Ralph’s site.

Best regards,
-- Al

Al, I was looking at overall output impedance that speaker generated EMF sees, and that would be double. 

I also stated that fully balanced design is not only unnecessary to provide great noise rejection, but in fact might be worse than one achievable in single ended amp with balanced input. High CMRR would require perfect gain matching of both "legs" of the amp and that is not possible at the level of good instrumentation amp (90dB @60Hz in my amp).  It would require 90dB  (0.003%) gain matching with discrete components, calling for <0.0015% resistors, that don't even exist.  It gets even worse at higher frequencies where both multistage amps/legs would have to have identical frequency characteristics.  Same is of course true for instrumentation amp, but to much smaller degree.  Resistors are laser trimmed on the same substrate while amplifier's bandwidth can often be much higher (22MHz in my amp).

Al, I was looking at overall output impedance that speaker generated EMF sees, and that would be double.

Thanks, Kijanki. When I commented about the doubled output impedance question I had lost track of the fact that you were addressing power amplifier outputs. Yes, **everything else being equal** the load applied by an amp to back EMF from a speaker will be numerically doubled (corresponding to half the damping factor) if the outputs are balanced compared to if they are single-ended. Although in the case of most solid state amps, at least, that is unlikely to matter since their damping factors will be sufficiently high for most speakers anyway. Notwithstanding the belief among many audiophiles that, for example, a damping factor of say 500 may result in better bass control than a damping factor of say 50, everything else being equal. I know from many of your past comments about damping factor that you would agree with my disagreement with that popular (mis)conception.

Regarding your last comment, I believe you are envisioning a balanced amp configuration that is used in some designs in which each of the two input signals in the balanced input pair is processed through its own signal path throughout the amp, that is independent of the signal path through which the other input signal is processed. While I believe Ralph is addressing the configuration used in his (and other) designs in which each stage of the amp, including the input stage, is a differential amplifier that responds to the difference between the pair of signals its inputs are provided with. And I believe that your comments and his are each essentially correct in the context of the kinds of designs each of you is envisioning.

Best regards,
-- Al