There will be some disagreement, but I think the best results are when you have balanced xlr all the way.
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I own an ARC LS5 MK II preamp which was the first truly balanced preamp from ARC. The LS2 and LS3 had XLR connections, but neither was a truly balanced unit. The LS5 has an awesome midrange bloom that is really second to none in the ARC lineup, new or old. I don't think this has much to do with its balanced design though. But the LS5 does not have a single RCA connection, in or out. There are no conversion circuits, phase splitter or diff amps, to degrade the signal as if it were to go from balanced to single ended or vice versa. There are so many preamps out there that do this and thus contribute to signal degradation. I talked to an ARC engineer last year and asked him about using balanced or single-ended cables with the newer preamps like the LS25 or Ref2. Both of these units handle both types, but since both are truly balanced designs, going balanced on the cables is the way to go as the + & - lines float above ground and thus less noise. ARC tube amps are inherently balanced input by nature and thus adding a single-ended input on them would only degrade the signal. Notice that ARC does not put single-ended inputs on their top end Ref amps. As for using an unbalanced source, well of course some conversion is done at the input of the preamp like the LS25 or Ref2, but then the signal is balanced from then on if the balanced connections are used from there. So still going balanced from preamp to amp, especially over long distances, can be very beneficial from a lower noise and thus higher resolution standpoint as well as not going through all these signal degrading conversion circuits if the circuits down the chain are truly balanced. Both my sources, ARC PH2 phono and ARC DAC3 are balanced along with the VT130 amp. I think the truly balanced nature of the system has much to do with its incredible resolution. And something like an NBS Statement cable really shows this as well.
I do agree that the connection physically and the distance can be a contributing factor,however many of the designs I have heard with strictly balanced out to amp sounded worse than using the single ended,so I suppose many of these factors are part of the circutry. Also I have heard balanced all the way through from cd to amp and they sound night and day better. For example I listened to a krell(not my favorite)intergrated amp,I think the 300i or something and with this design they installed a switch so you can change between the single ended input and the balanced from the same cd player outputting both cables into the amp at the same time, so you could do a perfect AB test, and the balanced in just was killer compared to the single ended,but I did not have this experiance by just hearing the amp ran balanced after any pre-amp without the source going in balanced..So I dont know,besides the superior connection of balanced and the possibility of running 50ft with less signal degrading I dont see or hear any reason to run only balanced out of pre...