Audio Research, next upgrade?

Assuming that your amp and speakers are a good match (I have no idea), then I would suggest getting the best pre-amp you can swing.

Seems pretty hard to really improve significantly over what you already have, other than what Swampwalker mentions, which is if the B&W is really an ideal speaker load for a 100 watt tube amp - possibly not, but not sure about that. Other than that, I can think of a lot of people that would be pretty darn happy with you ARC array.

There are a few Ref5's showing up from time to time here on Gon. Your LS26 will still bring a good price. That might be a good change.

Actually the B&W (801f ) sounds great with this amp. A lot of people on here say that you must have a very large amp to drive these speakers, and they will indeed take a lot of power, even though they are rather inefficient (85db) they are an 8 ohm load. They will easily get louder than I care to listen at. But I prefer to listen at low volumes late at night. And yes I am very happy with my system, but you know how the upgrade fever goes, so I was soliciting opinions as to which piece would make the most notable improvement: source, pre-amp, amp, etc

It looks like a nicely balanced system. Enjoy it. If you feel like spending money, I'd suggest trying a vinyl setup.