The non-B&O part: Yes, in the Marantz surround preamp, to direct  the LFE to front speakers, you just configure it so the front speakers are "large", i.e. full-range.  And you configure it so there are zero (no) LFE speakers attached to its sub connectors.  That will route the bass to the 20's.  Without looking at the manual, I'm not sure if you must configure it so the surround speakers are "small", i.e. limited-frequency.  (The 17's are sort of middle-of-the-road, not as small as the typical satellites used for surround in a less-expensive setup, but obviously not as bass-capable as the 20's.  I guess I would use "small" for a first pass, but try it both ways.  It really depends on what frequency the Marantz cuts off the bass, and how much mid-bass it pushes to whichever channel is reproducing the LFE instead of the actual intended (rear) destination.  Maybe the manual will give a hint? [Edit: Sure enough -- set the crossover speaker selection to "Individual" and then pick the frequency -- but only if the Subwoofer Mode is set to "LFE+Main" *and* the speaker is set to "Small".  Clear?  Didn't think so!  Definitely RTFM.🙂])

As to using BL2 instead of BL19, since the Marantz supports Audyssey MultEQ XT32 *and* Sub EQ HT, I would say get the cheapest thing you can get away with and let it fix up both your room modes and the sub's frequency response all in one shot.  If there isn't enough bass for your room size, you can change your mind and get another one. [Edit: sub placement still matters if you go for a second one to use Sub EQ HT; see, for example: https://www.reddit.com/r/hometheater/comments/a0kwwu/audyssey_subeq_ht_sucks/ ]

Additionally, I do not believe you require a separate subwoofer if you have BL20s as they each have a 10” subwoofer. As trackbeo says, if you’re unhappy with the bass you’re hearing, you likely need to tell the Marantz that the front R and L channels are “large” or “full-size” speakers. That way, the Marantz will not employ a crossover and route subsonic frequencies to the LFE channel but route all frequencies including bass to the front R and L. Sweet setup! Good job! 

So I tried few things-

 set my front speakers to large. My observation was that while listening to music it was amazing but while watching movie I felt I was not getting enough bass.

So I borrowed friends Beolab 19 and I could see it was making some difference. I set the LFE to “ Main + Front”. However, I still felt it was not that great. I am not sure if it’s was my setting. I even increased the Gain on it.

Has any body tried subwoofer from a 3rd part I.e 12in subwoofer? I have found somebody selling 1000w from Martin Logan and definitive  at a good price .  Is it worth trying them with my setup?