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.

Some amps can have half the tubes removed and work fine (Atma-Sphere's, for one). ARC amps cannot.

I own a D70MK2 and I think it is actually a better sounding amp than the bigger D115! The other plus, which I think a lot of tube amp users forget...or don't truly understand, is that the more tubes, the more heat and more potential for failure.
Some of the monster ARC amps with what....16 tubes or more, are IMHO just ASKING for trouble!
Lostbears, you say your D70 was thin sounding!!! Wow, you must have had one heckuva mismatch with your speakers...BUT don't blame the amp for the mismatch. IF you have another chance to hear a well set up D70 in a synergistic system, I suspect you would take back your suggestion about the D70 vs. the D115.

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.