Way back in about 1962 I bought the Dynakit Stereo 70 amplifier in kit form. I used it 'til in about 1974 I replaced it with the Dynakit solid state equivalent. I also had the Dynakit FM tuner and pre-amplifier.
The only 2 changes I ever made to the Stereo 70 were to replace the vacuum tube rectifier with a plug-in solid state rectifier and add a device called a surgister; probably they are unknown now. The reason I replaced the rectifier tube was to reduce heat output, but it probably slightly increased the power output also. The surgister was connected in series with the primary of the power transformer. It added resistance to the primary for a few seconds before applying the full voltage. The theory was that it would increase the life of the vacuum tubes.
In all the years I had the amplifier, I never had any trouble with it; I can't even recall that I ever had to replace any tubes. I eventually gave it to my sister and some years later, she gave it to my brother who still has it. However, it's now sitting on a shelf and not being used. Whether it would work now, I don't know. It could be that some capacitors would now require replacing.
I'm a bit skeptical when people say that one amplifier sounds better than another, or that some minor change improved the sound. All high quality amplifiers sound alike unless they are over-driven to the point that they produce distortion.
From what I've read, tubes may last longer if power is applied to the filaments for a while before applying the B+. Whether there would be a convenient and neat way to do that with the Stereo 70 I don't know. However, considering that vacuum tubes can be hard to get and expensive, it might be worth considering.
Incindently, before I got my degree in Business Administration, I had a trade school certificate in electronics and have a pretty good background in Physics. I'm retired now and am toying with the idea of making a stereo amplifier using push-pull 6V6s to produce about 20 watts per channel, AFTER I move into my new house. I currently have no vacuum tube equipment and would really like something to show people what vacuum tubes are; many people have never even heard of vacuum tubes.
Here's a tip to reduce the likelihood of getting smoke or blowing fuses when testing equipment; perhaps others here already do it. Connect the equipment in series with a light bulb of appropriate size. A tungsten filament, when cold, has about 1/20 the resistance it has when hot. If there is something wrong with the equipment that causes it to draw excessive current, the light bulb will light up and limit the current and probably prevent damage or blowing the fuse. If there is nothing wrong, the equipment may operate almost normally even with the light bulb in series with it. That works better than replacing fuses several times before finding a problem.