That should not be a problem.
There are lots of phono preamps that have one or more aux input ports in addition to the built-in Riaa amp (they are basicly a switch).
In that case you do not need the ‘phono preamp/aka Riaa preamp’ that you mentioned in your first post.
Beware – but you probably know it – that some BG’s already have a Riaa preamp built in.
P.S.
On a side note – personally I would not bother about ‘tube’.
The Theatre has many options to ‘design’ the sound for the inputs.
You can make a sound setup just for the line input there pretty much to your likings.
However, that is just my opinion.
MM