When I upgraded from
my Alesis Monitor 1's to
my JBL LSR 28p's, I noticed a huge difference. It changed everything.
When I upgraded from my Delta 1010 to the RME Fireface, I noticed an audible difference. The Fireface sounded noticeably better.
When I finally got around to treating my control room, I noticed a difference. Not HUGE, but I can hear the bass better and my mixes now translate better.
When I got a Royer 121, I noticed a WORLD of difference between that and a 57 on guitars. Definitely different, and now after listening for a month or so, I like it better on Marshall cabinets. But I still use the two together. The combination is now my go to guitar sound.
When I built my SCA preamps (which to date has been my biggest single $$$ investment) it has made a difference over my A&H console pre's. Is it worth the $2300 they cost? Depends on my financial situation. They don't make as much difference as any of the other stuff, but they add something, and I like it.
When I got my Microtech Gefell M930 and plugged it into an SCA A12, it just sounds good. No artifacts, no bumps, just good, clear and clean. A noticeable step up from my SP C-1 into A&H pre.
I feel that my studio is now complete and all of the big buxx spending is behind me. I have some nice tools to work with and upgrading from where I am now is pointless.