Well this was silly as it was an apples to oranges comparison to begin with but even more so because the OP asked for a loudspeaker with a similar sound signature (read: frequency response) not a loudspeaker with similar psychoacoustic characteristics. Response D2's are a good choice. They can be a bit peaky in their treble but so but their a fairly large at 17" high and even though they're front ported they like a minimum of 1.5ft from front wall to keep their bass tight. @Old_Snake it's best to first consider your environment and where your loudspeakers will be in relation to it when starting your research. How "near" will your field be? If positioned only at arms length away you really should be sticking with purpose built nearfield monitors or maybe you can roll the dice on audiophile loudspeakers but an audition is a must due to the constraints of placement in typical nearfield setups.
Passive or active monitors? You'll find most nearfield monitors are active and for good reason. The idea is to design amplifiers perfectly matched to the drivers they will be asked to drive; typically with dual mono designs, biamped with dedicated amplifiers for each driver, amplifiers matched to impedance curve of monitor, ultra short signal-path, class-d has come a long way too! The latest crop of active monitors by audiophile manufacturers such as KEF, Elac, Dynaudio and old favs like ATC have proved this philosophy works quite well.
Don't know your budget but it will have to be healthy if you chasing Zeus-R. Pete's right to recommend a loudspeaker in the $5K range. Great as D2's are I'd look at something utilizing newer technology. Genelec's "The Ones" series - even just the 8331 - would be as close to Zeus-R that I know of. They won't pound out the bass but neither do Zeus-R's tbh. There are slightly more mid-centric sounding Genelecs, but if A/B'd against 8331 it would be hard to take them home instead.
Back to the loudspeaker/earphone pyschoacoustic characteristics comparison for just a sec. @Deezel177 can speak to this more if he so chooses but music is recorded to give the illusion of a soundstage (read: performers in front of you). When listening via earphones we encounter two issues when trying to replicate this. The soundstage now becomes a headstage (so right of the bat not as production team intended), but most importantly, an earphone only allows us to hear one stereo channel per ear where with a 2ch setup each ear is subject to a pair of stereo channels and it's this glaring difference that has the greatest negative affect on an earphones ability to properly image like 2ch can. But hey, maybe you listen exclusively to binaural recordings @NaiveSound?? : ))
As someone who's spent hours upon hours listening to studio speakers in a well-treated environment, I can indeed confirm that they image better than in-ears can. Mixing and editing always feels superior to me in those environments, with one of the advantages being the ease with which a palpable literal stage is produced all around you. The only advantage an in-ear has is superior intimacy (which is only an advantage if it's what you're specifically looking for) and more sensitive left-right balance. I use IEMs to make sure my left and right channels are perfectly balanced, because left-right separation is slightly more apparent on them, but speakers always have the upper hand in immersion, layering, finer-detailing, effortlessness, etc. Due to the radiating nature of speakers, sound waves can mingle and surround you, but not in a way that degrades precision. In fact, it replicates the sounds you'd hear in venues and live studios, to produce a more life-like spatial representation of the track. I love IEMs with all my heart and I spend infinitely more time with them than I do with speakers, but I gotta give this one to the good 'ol two-channel speaker.
Like w900? I wasn't thinking about quality though.. Just like to have the frequency range as a reference for comparison even though its not 100% accurate most of the time.
Well, it's not really a matter of the frequency range's accuracy. Whether or not the numbers are sound, they ultimately tell you nothing about the IEM's signature or quality. Companies like Empire Ears probably produce IEMs that can easily go from around 20-40Hz low to 17-20kHz high. What's more important is the linearity of that extension and how it affects the IEM's stage, resolution, physicality, etc. In my experience, EE's latest line-ups all carry pretty great bass extension, but treble extension is understandably better on the higher-end models. They're all still beautifully clear, but staging and imaging isn't as pin-point precise because of this.