The Marantz Sr-14, 18 and 19EX are the Marantz Reference line. My guess is that they have better build quality, better power supplies, and maybe even better innards. Other than that, one of their major differences is that they have un-powered surround-back channels, whereas the SR-8200/9200 have full amps for 6.1 and 7.1 respectively. I haven't looked at the differences closely, but the marantz webpage is very informative.
The SR-14/18/19 is the reference line. They are great a/v receivers that sound very close to separates.
The 8200/9200 are newer receivers that have more channel amplification with more bells and whistles. I have an older SR-19 that is not EX so some of these features my be in the newer SR-14/18/19EX line.
I have not heard the 8200/9200 at home but was considering replacing my SR-19 with a 7200. My friend bought a 7200 so I had it hooked up directly to my speakers and preamped at my home to test.
Overall the 7200 sounded good in my room for the money. It's power filled my room with ease and it had the familar Marantz warm sound to it. But, I can say that my original SR-19 smokes the 7200 in sound quality. The SR-19 is clearer and more dynamic in both stand alone and preamped (it should for costing much more). So I decided the extra channel/DPLII wasn't worth this upgrade path for me.
I think you can't go wrong with 7200/8200/9200 if you plan on using the internal amps. The prices for these models per options/power is great.