Can't comment on 818 but the SE upgrade (new speakers) was much more significant than I was expecting. Improvements in areas I can't imagine the 818 can touch. Apparently the 818 brings its own additional improvements but better EBA, the lovely new tweeter, improved imaging etc make the SE quite a step especially on the 5200SLs. If you want sources too but want to sit on the MQA fence a bit longer before investing in an 818 then an AC200 works well (look around and ~£800 will bag you a used one) and is low cost for inputs like TV and Sky. I use my AC200 MS200 and MC200 combination for 'serious' listening and G61R for everything else.
When MQA is out, I will evaluate the options with my ears ! Hoping G65 will have support as makes for a neat upgrade ....

Various from vintage to SE