(A) "HiFi" is a seriously misunderstood word sometimes... It really depends on the speaker, but generally, *real* studio monitors (not the little boxes that say "STUDIO MONITOR" on them, but the big, overbearing, colorful, full-range, wide dispersion, behemoths) in the average studio are little more than audiophile-grade HiFi components.
The practice of using small, nearfield, limited range, short-throw, narrow dispersion boxes as mains is actually rather new... And I really don't care for it a bit.
So again, depending on the speaker, I'd personally *MUCH* rather be using quality HiFidelity components over the vast majority of what currently says "STUDIO MONITOR" on the box.
OF COURSE: If we're talking about some cheesy bookshelf speakers from BustBuy or something, then yeah, save up and move forward. But if you're talking about something along the lines of a B&W DM602 (around $600 *for a pair*) or JBL S-38's (I've seen them for around $300 *for a pair*) then you're way ahead of the game, IMO.
And of course, your monitoring will only ever be as good as the room they're in... Bass trapping and lots of it.
(B) There are few things that add "depth" to a recording than "DEPTH." Think about that and chew on it for a while. Distance is your friend. And again, bass trapping... A bad room isn't going to cut it.
(C) Volume in the mastering stage - Yes, unfortunately, "loud" has become synonymous with "mastering" in many cases. As much as it sucks, it's true to some extent. That being said - Sure, there is some technique involved and certain tools may lend themselves to attaining "sheer volume" more than others. But the most important part of the equation is having a mix that can *handle* those levels. Starting with a really great and dynamic mix, that was babied with generous amounts of headroom at every single possible stage from start-to-finish, is going to be much more important to the final product. "Tracking really hot" is NOT the way to get your final product really hot. In fact, it's usually just the opposite. Tracking really hot overdrives the input chain which causes all sorts of nastiness (including lack of clarity, focus, depth, added distortion, spectral dynamics problems etc.