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

Must be newlyweds, eh? You are creating/building/assembling a system. Tell her buying clothes isn't a hobby either and go get yourself a cd player. Unless you're the unemployed student and she's the bread winner, then say yes ma'am and do as she says.
Marakenetz idea of buying her new jeans isn't a bad one, but it can lead to higher expectations. I don't want to have to buy her things with my every audio puchase. Generally, I don't complain about the amount of shoes and clothes she buys, and she doesn't complain about my audio system. That's our compromise.

Tell her she is right, it is not a hobby, it is an addiction - but you had a choice between crack and high end audio and you chose high end. Would she rather you chose crack?

Definitely go out & upgrade that source component asap; the biamping might wait awhile however.
Then when you're coming home late from the bar & chasing skirts, you can rationalize that it isn't a hobby either (just be damn sure that you're not "creating anything", but then again, you do need a hobby ... right?).

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.