Underpowered?

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

Update:

It was the amp.  After checking all connections and trying unsuccessfully to play with positioning and with the amp's EQ, I gave my local dealer a call and they graciously allowed me to borrow their MA352.  They also made me borrow a power conditioner promising it would make a difference.  Coincidentally, they had a pair of 702 S2 driven by a 120W Rotel amp, so I got to audition the speakers there, and they sounded glorious.  So this is what I did when I got back home:

1. Added the power conditioner to the MA252+702s: sound was still anemic at low volumes, fatiguing at higher ones. Dare I say no difference at all.
2. Hooked up the MA352 with power conditioner (PC+MA352+702s): massive, immediately noticeable difference.  Speakers came to life and all my issues with them resolved completely.  Sound is big even at lower volumes, tones have texture throughout the spectrum, no instrument is left behind, bass is very present, fast, impactful, on your face but not boomy or overrepresented.  Sound is natural and emotionally engaging, exciting.  This is exactly what I was hoping to achieve with the speaker upgrade.  Big smile on my face again. I already know I've hit home and this is likely my end-game system. And this before even playing with EQ, biwiring, room treatment, positioning, etc. Heaven!

3.  Removed the power conditioner from the chain: this one was trickier.  Multiple back and forths and testing multiple songs.  If there is an improvement, it is subtle and barely noticed on A/B comparisons. Next step is blind tests which I will do later today.
4.  Finally, and just out of curiosity, I tried the 683s with the MA352: this was a hard comparison as it had been at least a day since I last listened to the 683s, but wow they sound great.  Yes, the 702s are better at everything, but if I couldn't afford them, the 683s would definitely do. Does the MA352 drive them better than the 252? Possibly, but if it does, it's not by a lot.

I am still getting a sub (used REL S5/SHO), so I will report back on how much it contributes, but I can tell you, now with the MA352 I feel no need for a sub.  Maybe that will change when I try one, but if not, the REL will be a great addition to my secondary system/home theater.

I just don't understand why B&W would rate this speaker 30-300W when even 100W were not enough to drive it properly, and when they recommend themselves using at least half of the maximum power.  I learned than an amp wattage is not just how loud it can get, but that it affects the quality of the sound even at lower volumes when output wattage is waaaay below the max.

I'm so grateful for everyone's input, and special kudos to @erik_squires @ditusa @samac @mofojo @jpconer @styleman @sandthemall @dinov @steve59 for getting it right.

For anyone who's interested, here are a few songs I thought were useful during all my tests (particularly the first 30-60 seconds of each).
- Lithium (Nirvana) for thar punchy drum kick that comes in with the highhat
- Better living through chemistry (QOTSA) for the realism of the initial percussion followed by the broad, bass rich distorted guitar that fills the whole room
- Gagging Order (Radiohead) good mids and highs from a clean, sweet acoustic guitar
- In the cold, cold night (White Stripes) for separation and balance of instruments and the soul shaking, precise bass line
- Acid Rain (Liquid Tension Experiment): for a busy, distorded, very stereo, complex analog metal song with challenging separation
- Twentysomething (Jamie Cullum): nice acoustics that allows you to listen to piano and vocals before introducing punchy drums and bass

Thanks to everyone who contributed.  I hope others also find this thread helpful as I'm sure I wasn't the only one unknowingly underpowering my speakers.

Daniel

After a long weekend enjoying my dealer's MA 352, I went to return it today.  I also asked them to check my MA252 for problems, since many people here felt it should be able to drive the 702s. They confirmed the issue is just that the 702s need more power.  I wanted to audition the Michi X3, but they didn't have one in stock and they were very confident the MA352 is the superior amp.  So, I pulled the trigger and took the MA352 home.  I also bought a Panamax MR4300 even though I'm not 100% sure how much effect the power conditioner had on my setup.

The MA352 + 702 Signatures combo gets me very close to the best setups I have heard at dealers for much less money and before even playing with speaker positioning and room treatment. Let alone upgrading my DAC, cables, etc. I know I shouldn't say this, but I really think this is it for me.

Moral of the story: make sure your amp has AT LEAST HALF of your speaker's maximum rated wattage!

You guys are right and my conclusion was oversimplified and impossible to generalize. So let me try again:

Moral of the story: never dismiss a pair of speakers before feeding them at least half of their rated maximum wattage!

Also: "Details matter" - @jjss49 (and thanks for explaining why!)

I still don't understand why B&W would risk making customers (and reviewers) unhappy by rating these speakers at 8Ω / 30-300W. But what do I know...?

A few more thoughts about the MA352:
- The headphone amp is noticeably better than MA252's.  I looked into it and the MA352 has something called "Headphone Crossfeed Director (HXD®)" that the MA252 apparently doesn't.  What a nice, unexpected surprise!
- I dig the default EQ both at low and high volumes.  No urgent need to play with the 5-band EQ but it's nice to have it.
- I am happy to find that even low kbps tracks sound better.
- I know its looks are controversial, but I think it's one of the best-looking amps out there.  Yes, I even dig the fake green tube LEDs. I just wish it had the blue OLED display of the MA252 instead of the very 90s-y green one (why doesn't it??).
- I know there are better amps/speakers out there, but I'm so happy with this pair as a starting place to build upon. Maybe a DAC upgrade is next?
- I am curious to hear how much of a difference the REL S5/SHO will make in my setup, because honestly, I listen to it right now and I ask myself how much better can it get?? 

Daniel

@mattuk How did your amp do with the 702s2? I'm still enjoying my setup. FWIW I ended up getting the S5SHO and it was worth it.