Why purchase the 645v3 over the 630v3? Higher dynamics, higher sensitivity and greater listening height if you move around while listening. Of course, a subwoofer can be added to the 645v3s but you may never feel the need for one in a music only system. If you do decide on a sub, it will have to be a very, very good one to improve noticibly on the bottom end performance of the 645v3s.
The 645's hardware consists of dual 7" ScanSpeak carbon fibre cone midbasses and our own now legendary R45 Ribbon driver but the 645v3 seems to be greater than even the sum of these parts.
The R645 is the most popular loudspeaker we make and is a classic in high end audio. It's popularity stems from its ability to light up virtually any room with a deep, wide and detailed soundstage with virtually any type of music. Its price doesn't hurt either.
Stereo and home theater systems in medium to large rooms driven by anything from a cheap (but not necessarily dirty) HT receiver to high end Spectron and Krell amplifiers. A few feet out from the wall and a bit of space in a normal room will produce a great soundstage and dynamics limited only by your amplification and thoughtful concern for your hearing.
A sub is not required unless used for very high volume stereo in a large room or for home theater. For home theater systems the majority of users will want a very good subwoofer with the 645v3 since it can operate solidly into the low 30s and can be crossed over well below 45Hz. Owners frequently run the 645s full range with the sub as augmentation rather than replacement at low frequencies.
Place the 645s several feet out from the short wall, firing down the length of a rectangular room and you will have a superb soundstage.