Does subwoofer + bookshelve=full range speaker?

I just took a look at your system and all I can say is WOW, nice setup! Are you asking this question because your 801s are too big for your tastes or room or just curious about an alternative configuration? Your question has been one of the ongoing debates in these forums. It's not a dumb question at all and should inspire some great responses.

There will be those like myself that use monitors with a sub because of the great soundstaging capability and ease of placement of small monitors combined with the versatility that a movable/adjustable sub bass system provides.

On the other hand there will be those that prefer floorstanders because of the potential difficulty of tuning a subwoofer to match monitors seemlessly or they may feel that full range speakers offer a more coherent sound.

I've never had floorstanders in my system so I can't offer any valuable insights into their setup. I enjoy small monitors because of their ease of placement and because I can own more than one pair for about the same price of a good floorstander.

So is a monitor/sub combo as a good as a floorstander? I think it depends on what you're comparing and how well each is setup.

It can almost do it, provided that you can cross your sub in as well as the designer of your speakers. With full cognizance of all the parameters involved. This is unlikely. But, is a possiblity. The other problem is, that subs, in fact, are directional, and the location out of the line-source will make a difference, and turn your system into an "array" instead of line-source. Typically, this affects imaging, in this case in the bass region. This is not to say that you couldn't get an acceptable result, and many people are happy with the results they get.

"if you add a subwoofer( assuming a good one) to B&W 805 or Martin logan entry level model . do you get the same performace as N801 or prodigy or more because the subwoofer in many instance will go even deeper than the full range speaker?"

NO

"if not why not?"

The 801 has a different xover (maybe even using better parts) and it uses a different midrange driver (the surroundless ones whereas the 805's have a surround made of rubber). The Prodigy should also do more in the top-end and midrange than the smaller logans. The same tweeter in two different systems can sound different too. Depeding on the sub you choose you may get more bass extension than the bigger model of the same manufacturer, but it turns into apples v. oranges at best. There's also the phase issues.