Marantz PM14 S1 and Bowers &Wilkins 702

@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 

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. 

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. 

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.