This time of year, this advice seems to come up. The advice is: "Don't buy 'experts' gifts in their field of expertise".
Examples:
Dont' buy a golfer golf clubs, don't buy a fisherman a fishing pole, etc...
The only exception is when the "expert" picks out the gift for you, and writes down the specific name and model # of the piece, or circles a picture in a catalog, etc.
"Experts" usually know what they want, and most of the time, already have what they want.
A reasonable person wants you to have a gift that you will enjoy. I don't know how knowledgeable your wife is when it comes to bass gear, but if she bought you a bass that you don't really like, I'm guessing that she bought it based on the advice of some "sales weasle" that doesn't know you. Or, maybe that was the nicest 5-string that was available where you wife was shopping, and she went for the top of the heap.
There are many diplomatic ways of returning the bass. Maybe you could exchange it for a less expensive piece, and use the rest of the money to buy your wife a gift to show your appreciation for her thoughtfulness...
Also, we're not talking about an ugly sweater or necktie. This is a gift that cost hundreds of dollars. Don't you think your wife wants that money invested in a gift you're 100% happy with?
If I were in your place, I would tell my wife how much I appreciated the time, effort, and thought that went into the gift, but for the amount of money she spent, that would not have been my first choice in basses. Then, ask if she would mind if you exchaned/returned it. Also, ask her to be involved in the replacement. Asking her which color she likes better, etc...