how to upgrade 802 N?

i have 802N driven by krell fpb-600 and krc-hr preamp. the DAC is accustic arts and the digital frontend is SSD with lossless driven though sony bdp.

The simplest upgrade would be to add 800/801D but that would be very costly and without fun,and i wanna some DIY type stuff.So the options are
1. Roll off existing 802N passively above 120HZ with ezxisting i am not sure at the moment to relieve the 8inch drivers of LF and improving clarity.

2.And purchase 801 drivers from B&W 15inch in custom made cabinets to operate between 120 HZ to 40 HZ , can use dsp to obtain linear response with separate amp.

3.And purchase ASW 850 15 inch subwoofer drivers from B&w in sealed isobaric pullpush configuration to operate below 40Hz with equaliazation?

any suggestions....or is it just toooo much to do.i just want highest quaity out of this world mid bass and bass and am confused that a single 15 inh driver should not be producing sound upto 320 HZ as in 801 B&W .....please comment
I also have the 802n but powered with pass labs x350.5. I added a subsonic correction unit from spacetech labs. This unit allows the 802 to go much lower than it's specs without damaging the speakers. You can read up on this unit if you google it.
Get a decent subwoofer, a crossover and a subEQ. That way, you will get better bass extension and power and not have to butcher the 802Ns.

It's really hard to integrate a subwoofer properly into 2 channel music. I had a jl fathom 113 and i could always tell where the bass is coming from even though i did the room correction. I think the only way to properly integtate the subwoofer is to use tact equipment.
Yeah, well something on the order of the TacT or DEQX would do it.

Get a better speaker. Brwmcab...Nope
You can use the velodyne SMS-1 to equalize the subwoofer properly. If you are hearing the subwoofer, then it is not integrated properly.

Some things you want to make sure you have set properly when dialing in the sub is volume and phase. You should be able to hear the subwoofer when you crank up the volume of the sub separately for added kick; for parties or just to jam out. You should never hear the sub when it is on and you are playing your music. If you do, then addressing subwoofer placement should be considered. I have my sub aligned on the same plain as my speakers, firing from right to left.

For an example, I was playing some music the other night and was amazed at how deep the bass was in this song and how punchy the bass was, Children of Sanchez (LP edition), Chuck Mangione, and when the tom toms struck, man, I was in awe, the bass was deep and articulate, I never perceieved the bass originating from the sub. I switched the sub off, and did not perceive a loss of localized bass, it just seemed that the system could not reach deeply into the subsonics, but the all tube based unit, with KT120's, still shook my room, driving B&W N800's, without the sub. Yes, some of that is system dependant, but, I integrated my sub using the SMS-1 and that device has worked miracles in regards to integrating the sub. I use an ASW850 sub with my system.

Anyone should be able to integrate a sub properly given time and the proper tools. Even without an outboard EQ, setting the volume, and setting the phase 180 out, may make a big difference.
If your goal is more bass extension, then add a high quality sub and integrate it properly, either with EQ or without. Kal is right on - messing with the 802D sounds like a potentially costly mistake!