The bx-300 was the first to depart from the older transport design in an effort to lower manufacturung cost. It is still made to operate the same way and with the same specs, but it uses different parts, and is also sort of a transitional transport to the later cr series. The whole bx series seem to suffer from idler wheel issues, but the transports still seem to work well. The bx-300 was a lower cost 3-head, and still I think offers great value for the 100-200 you can get them for. The electronics seem to sound good too.
The 680 series uses an early transport that some consider the best ever made. There are some that consider the 680 series the best naks, but they are less acknowledged because of the zx series. The 680 series had many models that I can't recall specifically, but some had some features like self adjusting record heads and auto calibration in some parameters. The 680 series was the state of the art at the time. While some of the self-adjusting features were gadgetry more than performance, they do not have an affect on the capability of them. The transport is REALLY excellant as far as operation and working on. The electronics are excellent sounding, and I am thinking that you would have to have one side by side with the zx series to decide which was better. As far as that goes, you really could be splitting hairs. There are many slight design changes in all the years nakamichi made basically the same deck, some better and some cheaper.
Aside from the transport, the electronics are tuned. While most sound the same in the context of a system, I have yet to experience 2 naks that sound exactly alike in the context of making a tape and playing it on another system. Some of this is due to the tuning of the electronics, and some due to the design and voicing. I would generalize them as this- the older 580, 680, 480 series as more revealing, detailed, and anylitacal, and the cr, dr, and later as more weighty, punchy, and lush sounding. These are more forgiving of high frequency loss as well. The zx, lx, and dragon fall in between with much of the best of both extremes, being capable of extreme fidelity while at the same time not as touchy. Also, in the early ones, model #'s ending with a 'z' seem to differ from non 'z's' in that they sound closer to the zx series and dragon, especailly for playback. To some, this may be splitting hairs, but some may really tell the difference. When you consider the tuning aspects on sound quality, as well as the context you would use the deck, some might consider the bx-300 close to the dragon. My personal favorites are the 580 and 680 series, tuned PERFECTLY, as I feel I can get just a little more from them than the zx series. (I am currently using a 582 as my main recorder, because my favorite 582z is out of tune!)
Hope this helps and is interesting.
I don't know what the mfsl geo tape is, but if it is an azimuth alignment tape, or has an azimuth alignment recording on it, it would be excellent. I use a nak azimuth tape which has a -20 db 15khz tone recorded on it to get the play head in close alignment. I then record a 15khz like I am adjusting for bias using the built in test tones on the deck. I then flip the tape over and record the same without readjusting anything. If it is within 2 db on the meter, and hopefully one, and not dancing around more than one db, that is good. If anything changes in the alignment, the first to go will be the azimuth, and the meters will fluctuate as the tape goes out of the critical range, and looks like flutter. There are variations in tape shells, so flipping it over doubles any variation. I can get more into it if you like, but I have been long winded enough, time to post.
