Stereo Preamp suggestion

therefore, you might look for an integrated 2 channel amp with pre-out/main in AND HT bypass

OR

add the preamp with HT bypass, use your existing 2 channel amp, and 

change the subs to a pair that accept speaker wire in/out to use your existing amp, thus the subs are always on, music or video, and the subs are volume controlled by the AVr in video mode.

For video, sub(s) location is less critical, however, using subs for 2 channel music, I recommend a stereo pair of self-powered subs, located adjacent to the main fl and fr,

to preserve imaging of bass. It is not only the fundamentals, ’.e. 36, but the overtones of those fundamentals 72, 144, 288 that give directionality. maintaining tight control for imaging is why I am not a fan of ports (mains or subs).
.......................................

note: line level: pre-out to sub, main in from sub gives BOTH the amp and the main speakers a break, no longer need to produce low bass. speaker wires to/from sub gives the main speakers a break, however the amp is still amplifying all frequencies.

Raven says low bass is 65% of the power needs, see their built-in high pass crossover option.

https://www.ravenaudio.com/product/blackhawk-mk3/

subs for 2 channel music.

HT bypass is an INPUT, for front left and front right line level signals to be volume controlled via the AVR along with the other surround channels the AVR is powering. Thus Video Mode Only.

your existing front amp does not have pre-out/main in, thus it is unable to feed the subs via line level at the same time it powers the fronts mains.

and the subs do not have speaker wire in/out 

are you powering your subs from the AVR now? that means they are only on when in video/surround mode.  

op

"Since the AVR is the only source for both music and movies, subs are on when the AVR is on".

yes, IF there is 'surround coding' for sub channel information; but, for 2 channel music, there is no decoding, there is no sub signal for them to work with, which means, they are NOT on for 2 channel music via AVR.

Preamp with HT Bypass would be engaged ONLY during Video, so yes, the front could thus be PASSED to the existing 2 channel amp, wired ONLY to the fl and fr.

btw, there is no center channel signal in 2 channel mode which makes it critical for the fl and fr speakers to create a 'wide' center image for off-center listeners of 2 channel music.