ARC D-115 use half of tubes

It is a very bad idea. You do not mention what speakers you have or how efficient they are. A 100 watts is not huge or even large for a push pull type tube amp. You want some headroom so the amp doesn't start clipping all the time.

But if you think the D115 has more power then your need, then why not sell it and buy a D70. Both the D70 and D115 are very similar amps. I had a D70 for a short while in the late 1980s. I sold it and bought a Classic 60. They both are rated at 60 watts per channel but the Classic 60 has twice the output tubes. I felt the D70 was under powered for my speakers and sounded a bit thin at times.

You have a very nice amp and I would not mess with it. I would replace the large caps though if they have not been replaced. Personally I would much rather have a D115 than another D70.

I had a mk1 version as I remember. It was a very nice sounding amp but seemed to run out of power very easily. Actually I sold the amp to a buddy who I believe still has it packed away somewhere. I had a pair of monitor type speakers at that time, I don't think they were terribly inefficient. It was 25 years ago and I just don't remember the model anymore.

As I said I replaced the D70 with a Classic 60. The Classic 60 was also rated at 60 watts but never seem to run out of power or sound thin. I later bought a pair of Soundlab Dynastats which I am sure were far less efficient. The Classic 60 still never seemed to run out of power in the same way. Granted the Classic 60 is a Hybrid design and not a pure tube amp.