Hello, welcome to the forum.
First, welcome the growl. It's a communication, and communicating is important. She is telling you that something is making her uncomfortable. You don't want to stop her, that would be like taking the battery out of a smoke alarm - the noise might stop, but the consequences aren't what you want.
That, I'm afraid, is the easier part. The other issue is the reason for her growling. It sounds like it could be rooted in resource guarding (guarding your wife, the bench, her bed). Given that she is a crossbreed that includes cocker spaniel, I think that's likely. Partly because cockers are known to have resource guarding tendencies and partly because dogs used to produce crossbreeds are seldom tested for temperament before being used in breeding.
I suggest your first step is to get, and read, the book Mine! by Jean Donaldson. But, since she is so young, and the guarding is sometimes over relatively low value things like the space under the bench, I strongly recommend you bring in a behaviourist who can assess her in person. Your insurance may cover this.
Behaviour work is an unregulated profession. So, you need to be really careful who you bring in. Anyone using a heavy handed approach will make this worse, as resource guarding is often rooted in insecurity, fear that something 'precious' will be taken away. This organisation may be able to help.
The Association of Pet Behaviour Counsellors:Excellence in Animal Behaviour & Welfare The Association of Pet Behaviour Counsellors (APBC) is a network of friendly and professional practitioners who collaborate extensively with vet-led teams, focusing on a variety of species such as dogs, cats...
www.apbc.org.uk