I own the ARC Ref 40 and Ref 10 phono and have alot of experience with tube replacement and tube rolling.
First, 100 hours should be plenty of time for burn in just about any tube.
The 6550s are in the power supply which is part of the signal path! Changing these tubes will make a very audible difference.
You have many choices of tubes including KT-88s.
I do not like the Sovtek 6550s as they are not as warm sounding and sort of steely in the highs.
ARC only changed from these as the SED Winged "C" s they were using declined in quality after 2010 and then ceased production. You can still find these and they are a big step up from the Sovteks but look at the date codes! Last production had a high failure rate.
You might want to try the newer Shuguang Treasure KT88s. These are very nice sounding tubes.
I have not tried the Psvane KT88s, but they are also supposed to be very nice.
The ARC preamp auto biases these, but I will note that trying to get matched pairs requires dealing with a reputable tube seller and isn’t all that easy otherwise. Forget eBay if you don’t have a tube tester.
I have not tried the Tung Sols but the NOS gray and black plates are extremely expensive. I have not tried the new ones and probably wouldn't bother.
Oh, it’s really worth trying Herbies titanium tube dampers on these. Reduces a little bit of grain in the presentation.