Well you can put a tube buffer device in the tape loop to get some tube effect. That is a good place because you can easily hear the difference in an A/B comparison of its effect on any particular recording/source. It would be my preferred location.
You can also get a seperate tube phono stage and just plug it into one of the AUX imputs of your reciever, but before doing so you will be able to hear the recievers phono stage (if it has one) sound with the tube buffer you put in the tape loop. Seperate tube phono stages can be difficult and expensive. One would not be my first choice unless it were going to be a major source.
FWIW
You can also get a seperate tube phono stage and just plug it into one of the AUX imputs of your reciever, but before doing so you will be able to hear the recievers phono stage (if it has one) sound with the tube buffer you put in the tape loop. Seperate tube phono stages can be difficult and expensive. One would not be my first choice unless it were going to be a major source.
FWIW