Marantz PM14 S1 and Bowers &Wilkins 702

I believe the Rega P8 or P10 turntables with the right tonearm & cartridge are step up sonically from the Marantz TT15 S1 turntable. The Linn LP12 is even better especially the LP12 Klimax version or package but are really expensive. 

However, Clearaudio makes awesome turntables. No doubt. 
I've heard the Clearaudio Statement turntable with an Audio Research Reference 10 phonostage and it was phenomenal. 

Kronos also makes spectacular turntables. I happen to own a Kronos Pro turntable with a 12" Black Beauty tonearm, Air Tight PC-1 Supreme mc cartridge, class A linear PSU and the Audio Research Reference 10 phonostage pre. It is definitely a step up sonically from the Linn LP12 Klimax version turntable package that I had previously. well....I still have the Linn LP12 Klimax turntable. 

Your turntable is a great piece for its price. It is not made by Marantz but by Clearaudio in Germany. Marantz sell it for $ 1,500 but the equivalent model with Clearaudio name sell for over $ 2,000.
McIntosh also sells this table under its brand for a lot more.
The TT-15 is a great table and the tonearm is also really good. I have a TT-15 for several years,  I just replaced the cartridge and it sounds really nice.
About the PM14, also agree it will drive the B&Ws very well. 

@sikhosanam 

IMO the Rega turntable will be good choice for your setup and system and will perform and sound way better than your Marantz turntable. Rega turntables start at little less than $1k up to around $10k not including tonearm, cartridge, PSU & phonostage preamp. Any of these Rega will sound better than your Marantz turntable.

Or you can look into a Linn LP12 Majik version turntable package which is an entry level Linn LP12 turntable version or package. Sonically this will be a step up from most Rega turntables. 

@sikhosanam 

You should upgrade and get a better turntable, tonearm, cartridge, PSU for the turntable and add a good quality external phonostage preamp. Marantz does not make good turntables and its accompanying accessories.

Rega makes great turntables at various different price points that are better than your current Marantz turntable.

There are good phonostage preamps out there. You can connect the phonostage pre to your linestage preamp, which in this case your Marantz PM14 S1 integrated. 

Linn LP12 is another outstanding turntable but will cost much more than the Rega. There are 3 different versions of Linn turntables : Majik version being the entry level one, Akurate version being the mid level one, and Klimax version being the top of the line version with top of line tonearm, mc cartridge, external PSU (power supply unit) for the turntable and top of line phonostage preamp all from Linn. They are very expensive especially the Linn LP12 Klimax version turntable, which will cost up to $35k and up. 

@sikhosanam 

The B&W 702 S2 are pretty efficient speakers and aren't hard to drive but their impedance will drop to around 3 ohms at certain frequencies around 100 Hz and below which will make it hard for an amplifier. 

Depending on how big your listening room size is and how loud you would play your music. Your Marantz PM14 S1 will probably do just fine. If you want to add an external power amp for your B&W 702 IMO the Rotel RB-1582 Mkll will be a perfect choice for your speakers. Rotel and B&W have great synergy together and are meant to be paired together.
I would also highly recommend the Classe Sigma Amp2 stereo power amp for your speakers. Classe & B&W also have great synergy together. 
B&W voiced and tuned their speakers using both Classe and Rotel amplifiers. 

The Classe Sigma Amp2 is a better amplifier than the Rotel and it retails for $3500 while the Rotel RB-1582 Mkll retails for $1600. These two amps have same power output ratings 200 wpc @ 8 ohms and 400 wpc @ 4 ohms on the Classe Sigma Amp2. The Classe Sigma Amp2 is a class D design and is Classe's entry level series, whereas the Rotel RB-1582 Mkll is a class AB design.

Or alternatively you can look for a used Classe Delta CA-2300 stereo power amp in after market like here on Audiogon. This amplifier was discontinued last year and can be had at discount nowadays.
The CA-2300 is a class AB amplifier and is a Delta series amplifier from Classe and sonically and performance wise will be a step up from the Classe Sigma Amp2. Its original retail price was $7k but since it has been discontinued nowadays you can find a used one for maybe around $4k or less. The Classe Delta CA-2300 is a superb sounding amplifier. 
I highly recommend it for you and it will sound great on your B&W speakers.

Anyways, you might want to try driving your B&W 702 using your Marantz integrated first and see how it is. You might not need an external power amp after all. Your Marantz integrated is a Marantz Reference series line and would sound and perform better than regular cheaper Marantz line. 

Btw, the B&W 702 S2 are awesome especially for the money.