The trick is to creep on these things by saying things like.....yes, I am going need to upgrade this receiver in the near future. I'm not going to it today but I want to say, it is "needed" to make things right with what I have.
Repeat the above in various forms and resistance wise, it softens the wife.
Yes, wife of mine, the credit cards are paid in full, the car and home needs have been taken care of. You enjoyed the visit with your family. It's time to get that AVR I've been going on about for the last six months or year.
Slowly the resistance melts away.
If you bring the upgrade up, out of the blue, then it's like an financial explosion but if it's brought up in the light of ongoing expenses, and has been mentioned many times, she might even say something like: if it will get you to shut up about it, buy the stupid thing. Where upon, in the heat of the moment, you say......"Okay."
The key to successful spousal negotiation is to make it their idea.
"But honey, you told me to buy it."