A:Answer Hello, Anonymous.
The answer depends on your room's size. While 10 additional watts/channel will make a difference with fairly large rooms, most HT rooms are of a standard 14' x 20' size and 10 more watts won't make that much of a difference there. Unless the room is in a noisy environment (you'd have more problems than just watts/channel, in that case), you won't miss that difference. However, the big differences are in other areas. For instance, if you have 4-ohms speakers, the TX-NR646 is not certified for 4-ohms speakers, whereas the TX-NR747 is. Another strong point is that the TX-NR747 is THX certified which will make a noticeable difference for your movie/gaming experience. THX performs over 2,000 tests on a receiver before it receives its certification and the resulting modifications to the receiver's delivered soundfield is superb. The THX Loudness Control feature alone would be worth its weight in gold for customers that live in a condominium, townhome or apartment, as that setting ensures that loudness levels can be set to where they won't make neighbors knock on the wall and you can still hear dialogue in movies clearly.
There are many other differences between the two models but those are a couple of the main points.
