I'm really just not interested in any single item to improve quality, and that should perhaps be 'quality', as so many so called quality improvements are tonal in nature. I have decided that real quality improvements need to be across the board, and the real killer improvement is not technical at all, it's the space. So many good microphones that could make a quality difference, but can't, because the NEED a good space to work in. Monitors that can actually reproduce the quality differences, and for me, at my age, ears that can hear the magic extra frequencies you are recording. Budget equipment is so good now, compared to twenty years ago that we should have a quality difference that has got better by a big jump, but it hasn't. When we talk about 'air' what exactly in technical terms are we talking? I have a strong suspicion we are just talking about the small differences a hump in the frequency response causes, and is that really a huge quality shift, when eq can do very, very similar things. Buying a Neumann U87 microphone is NOT, in my humble view an increase in quality, it's a positive jump in tone. In my studio, with OK monitors, and OK interface with an OK acoustic treatment and ok microphones, I would need to consider which one I could spend some money on to improve the quality of my output. Over the years I kept adding microphones expensive sample and synth packages and quality improved, but then topped out. I think that to improve my sound in a way that can be evidenced by the end product, I need to increase the size, improve the acoustics, and alter my mic distances. I can't put the mics back a bit, because the room intrudes, so maybe the better sound of my double bass from just an extra foot of distance cannot be tried. We all work within a constraint of some kind, and while money is usually the decider, we always have space issues, working from home.