30 foot long Balanced Cables to Balanced/RCA Adapters into Amp's Stereo RCA inputs?

There is nothing wrong with using balanced cables. 

Do you really need 30-50ft? Shorter runs are always better if you can - 10 ft and under for the lowest possible cable resistance.

Regarding jacks on any system, lighter cables with a solid fit are ideal. You don't want a very heavy cable. Some premium (very expensive) cables may be much heavier...

I would not use unbalanced runs for such a long distance, which is what you'd end up with if you used simple passive XLR to RCA adapters at one end. You're very likely to pick up a lot of audible interference.

You could use a transformer-coupled adapter, but that will affect the sound to some degree. Whether or not it is audible would depend on you and your system.

Alternatively you could use an active converter, which also runs the risk of having an audible impact on the sound depending on the converter.

Also, even for a shorter distance, you would need to know what the right internal wiring should be for your XLR to RCA adapters, unless you're using a transformer-coupled adapter or active conversion. You may need to ask OPPO or find someone else who knows the right configuration.

Thats very long you will loose some sound quality however with adapters.

For a run that long you'd really be well served to run it balanced. A regular XLR->RCA adapter at one end defeats this. So while not what you want to hear, if you really need to run that long you need electronics which accept balanced as well. One of the great virtues of true balanced connection is that long low level cable runs work well with a lower noise floor than thru RCA.