The X73i and X73 sound identical...the main differences are 1) EQ points [the regular X73 has more points in the highs, the X73i has more points in the mids] 2) The X73i has a 300/1200 ohm switch...the more expensive X73 does not. The 300/1200 ohm switch will yield more variety as different mics have different frequency curves and distortion amounts depending on the impedance input of the preamp.
The X73i and X73 use different switches also (the things sitting directly behind the knobs on the front panel). There isn't a "sound" difference between a Greyhill switch and an Elma switch. It's like saying a car drives different between Goodyear and Firestone tires, there are 99.9% more factors involved that effect the sound, and in this case they are fundamentally the same. Differences between the Vintech and Great River: It has been my observation that the Vintech "X" series (which is their entire line EXCEPT for their 1272 model) has lifted 'air' in the highs, where the Great River has a more linear/flat top overall.
Which is better? Of course that's subjective depending on what you personally and ultimately want to hear as being a dominant characterstic on your tracks, and both will yield pro results. Try them both is my advice, because they are very different. With the Mics...I think the 121 complements the Great River best, and the R84 complements the X73i the best. The 121 has a brighter more upfront sound (there are more high mids, less lows, and a scoop in the low mids as compared to the R84). The R84 is the opposite...far more low end, a more linear & flat midrange (from low mids to high mids). I've heard people say the R84 has more highs, but I don't hear it, they both have a steep rolloff above 8k or so. Just my personal preference though, hope this helps.
