Before I answer your last question, I need to be sure that I understand your present new setup that is working the best so far. Is there anything between the AC7 and the input of the Roamio other than the coax?
AC7 > coax ? ft > input of Roamio > TV
or in greater detail:
Code:
91XG > LNA 100 >
\
AC7 > Roamio > TV
/
C2V >
Are you trying to power the preamp thru the AC7, or doing it locally at the preamp location with its AC adapter?