Here was happened when I demoed them...
I am not an experienced audiophile; have minor hearing loss (due to excessive exposure to bomb blasts/gunfire/helicopters); and am extremely sensitive to high pitch sounds so please do take my findings with a grain of salt. The following was how I came to choose the 750s over the 650s/850s/1200s.
I used the following recordings (all in 256/320 Kbps AAC format... I am unable to distinguish any difference in bit rates higher than this) and listened to them in this order for nearly two hours when comparing the JVC HA-FX650 /750/850/1200 IEMs with one other:
- Strauss - Also sprach Zarathustra, Op. 30: Prelude (Sonnenaufgang aka Sunrise) - Karajan/Berlin Philharmonic; 1973
- Mahler - Symphony No. 2 in C Minor "Resurrection": I. Allegro Maestoso; Rattle/Berlin Philharmonic; 2010 (first five minutes only)
- Jarre - Oxygene, Part 2 (first two minutes only)
- Daft Punk - Sea of Simulation and Derezzed (Tron: Legacy)
- Robert Plant - In the Mood
- Paul McCartney - Every Night (Live on MTV Unplugged)
Please note: The HA-FX650s didn't even make it past the Also sprach listening tests... the details just weren't there... no where close to the rest of the FX line so I abandoned them.
Sound Quality Overall the 750s seemed to be warmer (more natural and detailed) and pleasant sounding to me... the lows were cleaner (clearer) and the highs very natural without any shrill or distortion... much more so than the 850s and even the 1200s. I honestly felt like the bass was more controlled on the 750s and therefore more powerful and cleaner sounding. It was must noticeable on the classical pieces especially with percussion, string, and wood instruments. I did not like the highs on the 850s at all and due to my sensitive ears the highs on both the 1200s and especially 850s actually irritated and bothered me and I immediately knew that I would fatigue easily using them. The biggest turnoff with the 1200s is that although they sounded the most like open headphones and offered what I believe is called an improved sound stage they were too hard for me to listen too... almost if they were pushing the music too hard... they were harsh; I hope that makes sense. Please feel free to correct me on that if I am using the wrong terminology. Timbre monsters.
Fit and Comfort The FX750s were perfect for my ears and remind me of the IE80s... the 850s and 1200s are however much bigger IEMs and felt heavier. The 1200s were simply too difficult to insert and fell out easily (I have small ears)... and this is coming from a user of Shure SE535LTD-Js!!! The 1200s were easily thrice as hard to find/secure a good seal than my 535s which til this day still require a few adjustments to find that perfect seal and fit.
Listening Volume The most important factor to me after fit/comfort... I found that I actually could listen to the 750s at a lower volume than the 850s and even more so than with the 1200s. This might have been due to the fact that I was listening straight from my iPhone 5 un-amped... though they are all very low impedance IEMs with high sensitivity???
Isolation There was no big difference between any of the FX models as they are all vented (however, due to their smaller size the 750s were easier to deeply insert into my ears and thus offered in my case a slight edge in isolation). The FX series are definitely not as isolating as my Shure SE535LTS-Js or SENN IE80s... with that said my first extended listening session with the 750s was on a seven hour plus flight from Bangkok to Cairo and they isolated well enough for air travel for me to use them again.
Cable Besides being smaller, one big difference between the 750s and the 850s/1200s is that the cable is not detachable. That is not important to me as I am very careful when using my headphones and actually like the fact that the 750s are even lighter due to a fixed cable system.
Carrying Case The provided carrying case with all the FX IEM series is an added bonus and is very well made and functional (albeit large and bulky it does protect your investment without a doubt).
Price I had gone in expecting to buy the 1200s so price wasn't an issue in my choosing the 750s... it just turned out to be a nice surprise in savings!!!
An Aside What really shocked me was how well the FX Wood IEM line performed with Ambient Electronic... I nearly melted my face off listening to Daft Punk's Tron: Legacy tracks. I am blown away by the completely different presentation of Jarre's work from my Shures. This really surprised me. I had figured that the JVC might play classical better but electronic as well?!