1. I wouldn't think that metal is overly demanding in terms of bass. We're pretty much talking about frequency ranges that home cinema enthusiasts wouldn't even fire up their sub for... midbass is important, but everything much below that quickly loses relevance. So competent coverage down to 100, maybe 80 Hz.
2. Small room and modest volumes presumably implies a rather short listening distance (<1 m)? Do you have normal to good hearing? If so, direct your attention to the background noise level produced by each model. Especially in the lower price ranges, finding some that aren't noticeably hissy is not a trivial task. The FX50's larger brother FX80 is supposed to be problematic in this regard (typical for inexpensive units with Class D amps and waveguides / coax), the Reveal 402 apparently is somewhat noticeable.
3. The size constraints suggest rather cramped conditions. Note that the best speakers in the world are going to have difficulty if just plopped onto a desk. Some effective EQ presets for such environments would be required at the very least. The KH80DSP allows for custom EQ, alas people are not exactly enthused with the app. The SC204 also has DSP capability but I'm not clear on how exactly that works.
A picture of the planned surroundings would be of interest, then we may be able to see how problematic the situation is.
Around here, I have my speakers on desk stands next to the monitor. That's why I'm essentially stuck with an old (but good) 5:4 19" job, it's about the maximum for usable monitor area. Any wider, and I'd be losing display height as it would have to be placed further back. Any higher, and my neck would be unhappy. Any more resolution would be mostly lost on my eyesight or rather lack thereof. Modern e-IPS panels would have substantially more IPS glow, a potentially major issue at my partially extreme viewing angles. At least I don't need my glasses like that...
