Underpowered?

Tricky situation and logical way out not always a help. If you was satisfied with your three way sound signature adding subs could not be solution. I would go with amp or speakers replacement. 

The first thing to do is if you have standard wires, you will need a jumper if there is 4 posts and you have 2 wires to connect( plus and minus)

Second, play some music walk around the room.  Do you hear boomy bass in some places and none in others.  Go to the corners, is it boomy there?  If you hear this then you should try moving your speakers around.  I point them right at me and sit back the same distance as the speakers sit apart.  Some speakers only need to be tilted in slightly.

Third, even if the room is not too bad for acoustics, a sub is a real helper here.  Very few speakers go down low enough  in the bass.  Especially, since you said you liked bass.  But you may find once you fix positioning and room problems your speakers are OK.  Try sitting a sub right beside a speaker.  Point it toward you or the wall.  If you have a second sub put it along the wall by you. 

Fourth, you are used to the B&W sound so I would not ditch these speakers...yet.  Same with the amp.  Depends on how loud you like your music.  But amp replacement is after you try a sub.  A powered sub has it's own amp and bass takes the most power, so your 252 may be enough.  Also, less excusion of the woofers reduces distortion of midrange frequencies.

Don't feel bad, they always say build your system around your speakers.  :)

I ran into a very similar thing but I knew that lack of power had nothing to disclose with it. Started out with what turned out to be an amazing thing I bought a pair of furutech wall outlet plugs all of a sudden I had some base out of my Levinson 33s. I then put a set of furutech plugs on the end of the captive power cords better again. Then I fooled around with room placement. Each time I got more base. Also thinking of you have two sets of binding post on the back of the speaker's it is time to replace the flat tin jumper bars and go to a true jumper. Easy to get that  before you buy just try it with some copper wire of a heavy gauge of you think it does a bunch then get a good one. Also I bought a cable break-in ma home that was made by siltech that made a huge difference on every piece of wire on the system. The tonal range got bigger at bother ends. It didn't matter if the wire had been used regularly for twenty years. The range got bigger the sound fuller etc. When you setup the room I personally like the speakers setup on odds vs evens. On evens you tend to accentuate mid bass. A real rock and roll set up the speakers a quarter of the way into the room and a sixth of the way from the side of the room and the chair a quarter of the way into the room from the back wall. Personally I set the speakers a their of the way onto the room and a fifth for the side walls. I Then.make fine adjustment from either one of those places. Make sure the speaker s are level to each other  and they are at the exact same height.  Do you have spikes down to the floor? Did you change any placement of the gear? Different stands can make a huge difference as well. Personally I think your base is there you just have to find it. 

Regards

The upgrade to MA352 will provide a modest improvement in SQ based on power, but dialing up the low bass EQ is a targeted solution for the better damped woofer alignment in the 702 Sigs vs. your old under damped 683s. Adding a REL sub instead of the amp upgrade is too, and depending on which model and what deal you can get on the Mac, may not be as costly. If your room permits various subwoofer placements that approach has the advantage of letting you optimize speaker placement for overall imaging, and sub placement for bass loading. The spot where the speakers deliver better bass may not correspond to where they image best. 

100 watts is plenty...BUT:

I think you need an amp that doubles down on the power with halving of the impedance. Or at least close to doubling.

Some amps will deliver 100 watts at 8ohms and 200 watts at 4 ohms and are stable down to 1 ohm.

An amp like that will not waiver with a big impedance dip.

You can also go further up the McIntosh line to amps with autoformers.

Monoblocks would help too.

And yes, I would also look at speaker placement.