OP,
I have all Audio Research gear. I find it is very synergistic. While I have experimented with different tubes on other equipment, I have never done so on ARC stuff… because first and formost the company has very carefully chosen every component to achieve a very specific sound. They have been working for decades to get this exact sound. Also, they use mostly contemporary made for “High End Audio” tubes… many not produced long ago.
Just for fun I called up a vintage tube fanatic… he berated Audio Research for the crap they made, and how terrible todays tubes are, and he was to call me back and give me some options. He never called back. This lunatic is highly regarded by tube aficionados. I spoke to a couple dealers that called him a raving lunatic.
That does not at all mean you shouldn’t play with tubes… go for it. I’d be very interested in hearing what you find out.