best place to introduce tubes into circuit


hello all
lets say my system goes:

source > DAC and/or preamp > amp > speakers.

where is the "ideal" place for tubes in this flow? currently have tube preamp in one configuration without DAC into class D amp.

all feedback welcomed, thanks!
ovaklakah
Post removed 
How about using tubes for the input/driver stages in a SS amp? My NYAL Moscode 150 uses a pair of 6CG7/6FQ7's per channel to drive a 4 per side set of MOSFET's. The best of both worlds!