without b&o sound system?
 

I'm interested with White / red combination (white exterior and red seat), and I found only one stock around my area. However, it doesn't have B&O sound system.

How much different the default sound system are from b&o? As i know, the default one is bose. and if i upgrade the system later, will the cost be much higher?

I had the default "Audi Concert" Bose system in my A5 and the B&O in my allroad and now TTS. If you're an audiophile you will definitely note the difference as the fidelity is, well, extremely faithful. If you have average perception, as do I, you'll still experience the richness of all that additional and more powerful hardware. The B&O is rather a relative bargain of the options available.
Having said that, the Bose system is quite good and especially if you typically just stream MP3 content you'll still be well satisfied. This is a premium brand vehicle after all and high end Bose components are utilized.
And yes, it's always quite expensive to retrofit a system of this B&O magnitude, especially given the additional speakers, etc. So, if you want B&O audio get it from the start by simply looking outside your area.
The bottom line of course is that this is very much an "ear of the beholder" selection and you should therefore hear it for yourself to properly compare. Listen to it in another model that is more available, such as an S3. The cabin volume will be different but still tuned for same.

The standard system is not Bose, it is the Audi Sound System, which is unbranded. If you want high quality sound, get the B&O and don't try and upgrade later. You will spend lots more money and this isn't like the old days when you'd just replace a head unit and get better sound.

Th base stereo is 155 watts and 9 speakers with a 5 channel amp, compared to 680 watts and 12 speakers with a 14 channel amp for the B&O. Best bet is to listen to them both and decide if the base stereo is suitable for your needs.

In my A5 "unbranded" Audi Concert System you could VAGCOM-on the Bose splash screen and custom equalizer settings. But splitting hairs.

Note also that the B&O is full surround sound capable with a very nice subwoofer (another difficult add-on).

The base system is good enough
 

Following are the data for TT sound systems:
Mk 1 Concert - 80W
Mk 1 optional Bose - 175W
Mk 2 Concert - 140W
Mk 2 optional Bose - 255W
Mk 3 Concert - 155W
Mk 3 optional Bang & Olufsen - 680W

Notes: Perceived sound volume depends on many factors besides wattage and includes crossover network performance and speaker efficiency.

To my ear, the Mk 2 Concert has noticeably better music fidelity as compared to the Mk 1 Bose but the maximum volume was slightly lower. I have the Mk 2 Concert in my roadster and it is just fine for my needs from a volume perspective and would be even better in an enclosed coupe. Mk 3 Concert should be even better fidelity and a minimal increase in volume. The concert is loud but if you are a fan of ear-shattering sound volume, the Concert is not going to satisfy you. I also agree with the other posters that the B&O is a sound system performance bargain. For me, I would prefer a car with the B&O option but would not make that a deal breaker because the Concert will meet my needs. Also, the B&O is not going to add much to the resale value. One approach would be to listen to the Concert in the Mk2 to see if it is good enough for you. If it is, you will be satisfied with the Mk 3 Concert.

Finally, I have never been able to get a TT equipped the way I want and/or at the price I wanted to pay at my local dealer. Your local dealer service department could care less where you bought your car so forget the old wives tale that service gives preferential treatment to local buyers. Make the deal in another city over the phone. Get someone to drive you there or spend about $150 for a plane ticket. If you fly, the dealership will pick you up at the airport and you can drive your prize home. You can also negotiate trades by phone and cell photos but I always do better selling it myself to a private party or a local dealer or CarMax.