Underpowered?

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.

KISS

All great advice (esp, placement) but regardless a sub or two will add significantly.

Wow, many thanks to everyone who shared their opinions and experiences! It's so great to be part of this community and I only hope others also benefit from this post.

I'm surprised to see that there are solid arguments for each of my 3 options and that this problem might have multiple solutions. It seems the most recommended changes are by far positioning and adding a sub, followed by upgrading the amp and lastly replacing the speakers.  I will start by trying the simplest/cheapest ones first.  BTW, I brought back the 683s and yes, they are fuller, richer, more balanced and with a nice low kick.  The highs aren't as detailed and the 683 miss some extreme high frequencies that are not necessarily missed in music.  Overall, I seem to like the 683s better... Could it be that speakers sound better after a few years?? I know the 702s have great potential, so this makes me think there's a problem with the 252+702 pairing more than the speakers themselves.

Another aspect of this is that the issue doesn't resolve with high volume, which yes, comes with a better bass response but at levels where the highs are too piercing and fatiguing. The amp and speakers ARE capable of getting to good bass levels, but not in balance with the rest of the frequencies. That to me suggests the amp is underpowered since I doubt the speakers were made to be this skewed.

I will definitely play with positioning and report back. The speakers are fairly close to the wall as is.  Exact same position the 683s were in and never had a problem.
The ports are open with no foam plugs (I'll try plugging them and moving even closer to the wall).

Speaker cables are going in the lower terminal with the metal jumpers that came with the speakers. The amp only has 2 output terminals.  Is there a benefit to cables that go from 2 to 4 terminals?

Break-in: I doubt that's the issue but I will continue to listen and hope for an improvement. I also found some youtube break-in tones focused on lower frequencies that I could leave on for a few hours.

The B&W advisor's exact words were: "The 702 Signature speakers are rated for up to 300 Watts. Using an amplifier that outputs at least half that (~200 Watts or so) will allow you to get great performance from the speakers." So I will ask my local dealers to see if I can borrow a amp that's at least 200W per channel (min wattage recommended by B&W advisor). I will also try a sub.

@phantom_av: interesting suggestion but i checked and there's only one pair of speaker output terminals labeled 4/8Ω.

I am not necessarily tied to the 702s, but I loved everything about the 683s and was looking forward to simply improving everything about them.  I still hope I can get there!

Thanks again to everyone!

Daniel