Stereo Preamp suggestion

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.  

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. 

Mcintosh has “passthru” on many of their preamp‘s. I’ve owned the C1000 and the C8. Both of work flawlessly

A budget number would be helpful.  I used a Modwright LS100 until recently and have since upgraded to a Herron VTSP-360 Reference.  Both have HTBP. 

My subs are active for both stereo listening and home theater.  Many preamps have more than one set of outputs.  You will give up the subwoofer correction from your AVR, but if you run enough subs (I have 4, 5 for HT), it won't matter.

I’m not 100% positive but I think for what you are trying to do only the parasound p6 or anthem str will actually do what you want. I believe they have HT bypass for the RL speakers as well as subwoofers. I had the P5 not the P6 and found it really to be not much better than the marantz prepro I used for HT in regards to 2 channel music. The anthem STR is probably my vote out of the 4 because it has the features you want and will sound better than the parasound but it is over double the price.

I have to agree with last two posters. The Parasound P6 is just want you want to integrate 2.1 with 5.x from AVR. I had the P5 and now P7. The subwoofer can work with both.  I use Roon and DSP for 2.1 and anthem avm60 arc for theater. Best of both. 

Par a sound.

Parasound P7 is worth consideration here, as many listeners, including me, believe that its sound surpasses that of P5, P6, or HALO integrated’s preamp section. The P7 has only one sub output, but you can use a y-cable to split that output for use with two subwoofers. Since the P7 is discontinued, Parasound cut the price in half, making it a great value too.

Check out the Rogue RP-1. It has two outputs, one you can use for your sub. It also has a really good phono preamp for the price point.

@elliottbnewcombjr 
thank you for the elaborate response. I intend going for the second option of adding a preamp in between the AVR and power amp.
as for subs (sealed ones) location they are adjacent to the L and R mains so that is not a concern as such.
From the posts so far I believe only P6 or Anthem STR would fit the requirement. Unable to decide which one to go for since STR has the ARC which is being raved about. 

@tony1954Thank you for the response. Yes I have read this link earlier
My source is predominantly Tidal, Spotify and youtube. Let me know your thoughts around Anthem STR if you have come across since it has the ARC and If i understand it right, it has the flexibility on tuning the subs suitably.

Rotel RC1590 sucks too much of noise and humming 
PARASOUND P6 is your best choice pair together with A21+

I tried P6 when it was first released.. find it pretty universal and functional but boring sounding. +1 for Modwright LS100 and Herron VTSP-360 Reference. 

I purchased a Linear Tube MicroZOTL about 8 months ago. I am very pleased. Prior to that i was using my Marantz AV8801 prepro as my 2 channel preamp. Adding the LTA to fill that role made one of the biggest positive changes to the sound of my system. Also, I found them to be a great company to deal with.

Anthem.... 'nuf said...

+1 on the RP-1.  I know mine has HT bypass but I have never used it.    But I did have a sub set up with it for awhile.   I have mine fronting a solid state class a/b amp and like the tube/SS combination.  Then you can have fun tube rolling. 

You can call Rogue and they will always answer the phone and run you through how set it up with your particular sub. 

They pop up used on here and Usaudiomart for around $1.2k but get snapped up pretty fast.

Absolutely love my Wyred4Sound STP-SE. Don't buy anything else without hearing this one. Some very nice reviews are out there as well.

Thanks everyone for the responses. Truly appreciated.
I am now deciding between Parasound P6 and Anthem STR as only these two fit all the requirements. While Im inclined towards Anthem, it costs twice as much as P6. Not sure if its worth the spent on Anthem only for the ARC and the stereo sub option which is not in the P6.
Further, since my source is Tidal and Spotify, Need to go for an audiophile grade streamer to go with my system.
Any thoughts would help. Im thinking of Node and Cambridge CXN V2 as of now.