Splitters may not be the ideal connection, but they are good enough to hear differences.
Like/prefer it? keep it? return it?
friend brought his new tube phono to compare to my tube McIntosh mx110z's phono.
TT to splitter
splitter a. to his new tube phono, then to integrated amp 'AUX'.
splitter b. to my SUT, sut output to mx110z mm phono in/preamp out to integrated amp 'CD'.
switch integrated inputs 'CD' to 'AUX', instant comparison.
NOTE: volume differences might have to be dealt with. In my case, I was able to adjust the mx110z's volume to match the volume of his into AUX. Sometimes it is just a temporary pencil mark.
In my case, 3 arms into/trru SUT with 3 inputs and PASS for MM, we were able to compare MC; MM and MONO MM. I needed to adjust volume for their differences.