Does having two subwoffers make a difference?

I will prorbably go for a 9.1 system
later in the year get new main speakers and use my current main ones for the rear


Thanks

2 Subs are definitely better than 1 but as has been said it depends on your room and what treatments you do etc. If you are just settings this up in a 'normal' room like a lounge then 1 is more than enough.

You probably know this but when you get new speakers (and you are going to use your existing ones for the rear) make sure you 'sonically' match them.  

Last year I built a dedicated theatre room in my house and there is so much to think about. www.avsforum.com/forum/ a great site for ideas and people are willing to give you some great advice.

The only thing I would say is don't buy a UHD (4K) TV yet. There is a new colour space hopefully being introduced later in the year called REC2020 (replacing the current REC709) and from all reports it's night and day difference.