NOS means New Old Stock. Older models but still new. Price=lower
Based on what I read (not my personal experience) Wharfes are better for Yamaha. If you are going to get the 1800, go for the mid level Wharfes like Evo's.
How much ba Rotel amps compatible with the Yamaha 1800 or is it really needed?
Really needed: no. But its a great upgrade path for more power & better performance. Also great for big rooms. Most high end 100+k receivers use torroidal power supplies used in power amps like Rotel. See the Yamaha Z9's power supply
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Rotel 5 channel amp: P56k. Rotel 2 channel amp: P34k
More about power amp advantages read reply 28
here