I really have started to smile when people start talking about 'quality' when they really mean 'difference'. It's so difficult to use words to describe what you hear. In my own head, 'quality' can be many things. If you record a grand piano, in the middle of a wind free wilderness, then will every key be recorded at the same level? Nope. At some point top and bottom it starts to tail off. If you use a different mic and squeeze a few extra notes out of the top or bottom, is that a quality improvement? Yes. In practice though, has anyone used any mic that cannot record the top and bottom notes? No. So what's the difference? Do the mics record a different waveform? Well, yes they do because the things missing are the overtones, and their presence is, in my view, a quality improvement. We talk about loads of other sound features with flowery language - openness, clarity, smoothness, transparency etc but we've moved on to the space it was recorded in now.
What do the cheaper interfaces do, compared to the very expensive ones? They're electronically noisier when the gain gets turned up. Some have detectable data noise in the background, reducing the signal to noise ratio. I've not had one compromised in frequency response. Some do sound more compressed than others - never really sure what electronic process causes it? Maybe something in the converters reducing the available bit depth. Certainly looking at full scale meters often shows stuff happening at the bottom of the range that while I cannot hear it, seems to be there?
I have 3 different interfaces I use, and none are blisteringly sophisticated and I don't think in a blind test that I can tell which one was used in a decent recording. Like the example of the SM7 and a quiet voice above - I reckon given that mic and scenario I'd make a good guess on the interface, because one of them is clearly weak used like that.
I'm also wary of the idea that a bass guitar sound has a 'quality'. I'm a bass player - have been since I was 17 and now I'm 60. There are quality differences in the construction of all my basses - and I have I think, 7 currently. They all sound different, some drastically so, but I cannot use the word 'quality' to describe the differences. The worst sounding bass - dull, muddy, poorly defined, difficult to pitch is an old semi-acoustic with flats that was de-fretted in the 80s still has a place. It has a 'sound', a kind of dated sound, but I cannot attach the word 'quality' to it. The cleanest and brightest sounding bass is a Fender American Standard Jazz, and it's horrible to play - but again, does the job. I can mic them, DI them and play with their tone, and I will change mic positions, swap mics and stick bits of gaffer tape on the basses if I have to, but I would never dream of trying a different interface. The tonal difference between them is so tiny, there's no point.
If I wanted to improve my sound quality - then it's absolutely microphones first. I'm not even sure that I'd worry about the monitors. I get Dave's points, but you can cope with less revealing monitors once you know them, and as long as they are reasonably truthful in what they produce, again, it's a tonal difference. So for me, it would be mics, room treatment absolutely number 1 and 2. Then I'd be looking at the monitors, because the first two impact recordings - you can upgrade monitors and then revisit old projects and tweak them. You can't do that if you start with excellent speakers and poor mics and room. All my interface changes have been made when I needed more inputs. I do appreciate many people are on a quest for the best, whatever 'best' is nowadays. Apart from the pre-amp link with low output mics, I really cannot hear the subtle difference other claim to. I accept they are hearing something that is different in a positive way, but I suspect their listening and recording environment is the only thing that makes these differences audible. In my studio, I just cannot hear interface swaps. I can hear mic swaps.