Except to note that most actual linestages confer some gain on the signal, as well as impedance buffering
Hi Lewm, yes the IO by itself doesn't really have the linestage gain. While, with phono input, the IO should have enough gain to directly drive the power amp, experienced users like Albert Porter commented that adding the accompany Callisto linestage to the IO would yield further benefits.
I have both the IO and the Callisto with me, but since I only got the IO recently, it is still running in. I would certainly compare the IO vs IO + Callisto combo once the run in is completed.