"I know the tape loop method will allow easy switching back and forth, but is it a true A/B comparison?"
This is a method to see how "coloured" a cable is. It may not be representative of how the cable actually works in a different part of the circuit with different mating impedances.
"Is the signal path and switching contacts of the tape loop cicuit the same as the line inputs signal path and switching contacts of the preamp?"
This would depend on if the output jacks on the source were wired in parallel or if they were individually selectable. If they are in parallel, it would not be the same. If they are selectable, it would be almost the same. The difference would be that you would have to select outputs on the source and the inputs on the preamp. This would mean introducing two switches and their associated parts / signal path into the circuit rather than just one. More variables with more potential for outside influences.
On top of that, signal goes wherever it can, even if the circuit is open and there isn't a complete path for it to take. This is especially true of RF based signals, such as the digital output on a transport that one might use to feed a DAC. I know that this is contrary to what many people have been taught, but there's nothing that i can do about that. Since the cables themselves present a nominal impedance, signal will flow and they will effectively become part of the "load" that the device sees. This is true even if you and i see it as an "open circuit".
As such, using a device with parallel outputs and choosing between the cables at the preamp still places the nominal impedance of the unused interconnects in parallel with the signal path of the interconnects being used. Nothing short of switching them out of the circuit at the source or physically removing them would solve this potential variable.
"And are you not putting the two ics in series with one another when using the tape loop".
My take on this was that the system remained as it was and one was simply inserting cables for testing into the tape loop. This would allow one to maintain the sonics that they were used to and at the same time, allow them to switch the interconnect in and out as desired via the tape loop. If one noticed a large difference, the cable itself would probably be introducing its' own sonic influence. As previously mentioned though, there would also be additional impedance related alterations to the signal path due to the tape monitor circuitry and additional switching, etc...
As to your specific installation, i'm not directly familiar with either piece of gear i.e. the Arcam or the SF preamp. How each piece works in terms of selectable inputs and outputs and loading characteristics is a mystery to me. Sean
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