How to respond to those who say it's not a hobby?

Try to audition some stuff with return privileges. Get her
input as to if she can tell a positive difference. Just a
thought!

Here's my take: What she is communicating to you is not "it's not a hobby". What she's trying to communicate (and has, it sounds like) is "I don't want you spending money on a new CD player and amp". After hearing this, you justified the purchase by saying it's a hobby, so now she counters with "technically, it's not a hobby".

Your secondary argument over the hobby-ness of audio is not the real issue. You're arguing over money, plain and simple. You're in good company, it's the number one subject of argument among couples. It's interesting that your better half sounded quite accepting of your non-hobby until the purchases crossed her comfort level.

It's not relevant whether audio qualifies as a hobby or not, there's no "true" answer to that question. I would stop looking for ways to convince her, as this usually creates the opposite result of what the convincer intends. My advise to you is to sit down with her when you both are in a good mood, and talk about money. Work out for your relationship what the limits are (if any) on purchases and how you individually can make purchasing decisions while still being a part of a couple. The time and effort you spend here will really pay off, as this subject will come up again and again. When she speaks, listen to her perspective and shut up. When she's all done, tell her you understand her perspective. Then tell her your perspective. Then when that's all done start finding solutions together.

You may end up being able to get all the electronics you want AND have a more fulfilling relationship at the same time. It's all about communication.

Tell her you are in a mid-life crisis, and you're either going to have an affair, buy a Harley, or upgrade your cd player.
She'll make the choice for you.

Timing is everything. If money is not a problem then go ahead and FINISH your system and plan on living with it a few years. If you do have a BUDGET and she feels it is not time, then wait. You really do not need to justify your interests in audio/music nor have her validate the interests. Hence do not actively seek her approval of your interests. You can involve her without allowing her involvement to dictate your level of involvement. That said, you do need to stay within the realm of whatever financial agreement you two have for discretionary spending. Timing, timing, timing! If it ain't time...wait.

There's probably more than one reason why most audio components are black. One benefit is the fact that to the untrained eye, they all look alike. All of my equipment has changed at one time or another and I never heard a word about it. Speakers on the other hand can present quite a challenge.