I owned the 801fs many years ago not at the same time as my 801 MS3. They were in a different house/room with different electronics so a direct comparison was not possible and the room is the most important IMO. My knowledge of the speakers back then as well was not what it is today. The early 801 up to the 801 matrix S2 were hard core studio monitors designed for minimum downtime. They all had circuits to protect them. The 801 matrix S3 was the first 801 model to go naked without any protection. I liked the head environmental controls to tune the sound on the 801fs. The 801 matrix requires this be done with cables and electronics now. Many times I have thought about buying the 801fs again as a project when they come up for sale at ridiculous prices.
If I had them as a project I would modify them initially by:
Removing/bypassing the protection circuit.
Implement a bi-wire capability.
Connect a tube amp like your ARC to the midrange/tweeter.
Connect a big SS amp to the woofers
I am very happy with mine and would never consider replacing them because I have NOT YET found their limits.
My electronics are still the bottleneck not the speakers. Whenever I improve something (source, amp, cables) they end up sounding better.
When you get them hooked up on the stands before you place them in their final position please listen to them in near field at least 5-6 feet from the front wall. Would love to hear your impressions.
Cheers