MC240 Tube Recommendation

I'm just purchased an McIntosh MC240 and was wondering what experiences people have had with different tube combinations. What combination of brands works best? Are NOS tubes really going to make a difference? I'm hesitant to spend so much on trial and error with tubes.
I changed 4 new output tubes are 6L6EH with Electro Harmonix for my MC 240,its made in Russia,and already make the sound different,which are inexpensive tubes,but recently,I changed it as mono block and changed the high grade speaker cables,it make the sound completely different.I will suggest change it as mono block rather than spent money for the expensive tubes.
To my ears NOS 6L6GC and 7027As are the only output tubes to put in your MC240. I purchased both Svetlanas and Sovteks and don't like the sound of either. My favorites are the RCA black plates and GE grey plates (6L6GC). I also
like the Sylvania 6L6GC. The GE and RCA 7027s also sound wonderful, but my wife doesn't like the taller bottles on the exposed amp in our living room (I agree). I have purchased enough sets of good NOS to last me until I'm well over 100 years old (or I guess my wife can sell them in the estate sale around the year 2044). Really, try the GE 6L6GCs, they are going for fair prices on Audiogon and Ebay.
Good luck.