Stereo Preamp suggestion

I am using the Marantz AVR for both music and movies. 
Fronts are connected to the Poweramp and yes the subs are connected to the Marantz AVR. Since the AVR is the only source for both music and movies, subs are on when the AVR is on.
I thought by adding a stereo pre, it would sound more musical rather than listening thru an AVR.

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).
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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/

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.

This may help.    

http://www.audiophile.no/en/articles-tests-reviews/item/426-amplifiers-with-processor-input

I was in exactly the same situation a couple of years ago and the solution is easy and extremely satisfying sonically.    
Adding a preamp with HT bypass will be a significant improvement, as it removes the Marantz's preamp from the system. Although you do not provide any source information, I assume you use either vinyl, CD or digital of some sort, so all 2-channel will run through the preamp. If you were to buy the Parasound Halo P6 it would solve a lot of what you are looking for. It not only has HT bypass and dual sub outs, it also has a set of additional sub outs with variable low pass filter and a variable high pass filter for the main outs.