MN,
> By 'broadband absorbers' do you mean slot resonators? <
No, I mean treatment that absorbs over a wide frequency range. In other words, something like fiberglass for the mids and highs, and efficient bass traps to handle a significant portion of the low frequency range.
> I do have one slot resonator built into one of the walls. <
The problem with small rooms is they need proportionally more bass trapping than larger rooms. If you cover at least 30 percent of the room surfaces with bass traps, you'll have a much flatter low frequency response than covering only 5 percent. You didn't say how large your slot resonator is, but unless it takes up an entire wall it's probably not enough.
> Is there anything that would take up less space like the typical foam with cones or ripples? What would be the most efficient method, efficient meaning space and frequency absorption? <
Foam is one of the least efficient absorbers, though it's fairly easy to work with. Rigid fiberglass is much better at all frequencies, and dedicated bass traps are better still for low frequencies. Not wanting to turn this into an advertisement, but have a look at my company's site:
www.realtraps.com
Especially the MiniTraps page. That will give you some ideas. Also see the FAQ article for my Acoustics forum:
www.recording.org/users/acoustics
--Ethan