Stereo system vs Steinway

Hleeid, I have my father's 1929 Mason + Hamblin parlor grand. Interestingly it has a Steinway pin block in it. Apparently at one time they were having trouble getting the right wood for it so they bought a bunch from Steinway. It is valued between 50 and 60 grand. Anyway, We live in New Hampshire and have wide swings in humidity as the seasons change. It is not so much the exact humidity that matters. It is a constant humidity. If the soundboard dries out too much it will crack and the piano will be worth 1/2 what it should be. Check the sound board carefully. Many of them have already cracked. They can be repaired but you can not replace a whole soundboard. They fill the crack and sand it down. 
A 10 point variation is definitely OK. Many pianos will tolerate a 20 point variation. I allow my house to drop to 35% in the Winter and let it go no higher than 55% in the Summer. Electronic equipment and speakers have no problem with humidity as long as nothing is condensing on them (high humidity with an air conditioner blowing on them) Records on the other hand do not tolerate high humidity. Over 80% and they will grow mildew and other fungus which smells bad and can ruin the vinyl surface. Enjoy your piano! 

There are humidification systems that attach directly beneath the piano to help- not a complete solution but doesn't require that you turn your room into a jungle climate. I had a couple of old concert grands- a Bosendorfer and an Erard. They sounded gorgeous when "voiced" but after a few weeks, started to sound sour. I got somebody who really knew pianos in NYC to come up to look at it. He used to do a very famous conductor's piano set up and tuning. He hemmed, he hawed, he looked, and then said with a thick European accent, "just buy a Steinway."  :)
Some of the vintage pianos are great; there was a shop near where I lived at one point where I just happened on a door in an industrial facility and there were countless pianos, almost all big grands, in rows, being torn down, rebuilt, etc. I happened on a place that was well regarded for restoration of these instruments. Never did get any of mine restored, and eventually sold them. 

Yes, the soundboard is the entire reason to worry about humidity on your Steinway, but you don't need to be obsessive about keeping things at or above 50% humidity.   The thing to worry about is too much humidity, and too broad of a range in humidity over a broad period of time.  

I live in Minnesota where we have big swings, and we run two evaporative humidifiers in the winter to make sure things don't drop below 35% for any length of time.

Actually, summer is the bigger worry, because it is the swelling of the soundboard with moisture that causes it to "stuff" up against the solid case, and that compression translates into some "set" of the wood, making it smaller.  Then, when things dry out in the drier periods, the wood can go into tension causing cracking and loss of camber in the board.  That's when your piano's marvelous tonal quality disappears. If it loses its camber, a re-build is the only remedy.

Millercarbon's advice about temperatures and humidity with a stereo is spot on.  Not much to worry about there, unless you are letting your house swing wildly with crazy A/C use or something...

@millercarbon - As always, I again appreciate your clarifications. Took my peace of mind up several notches.

@petg60/elliotnewcombjr/mijostyn/whart/jbrrp1

Your inputs have filled in a lot of the blanks for me. Respectful acknowledgements to you as fellow pianists.

@whart - Wow! Bosendorfer has been on my dream list ever since playing a few in downtown Boston decades ago.
Are there any direct attach humidification systems you have used/can recommend?

@mijostyn & jbrrp So yeah the soundboard has cracks but can be repaired. The piano is an original to the instrument 1931 Steinway Model M Grand. Fortunately, I hooked up with an awesome former Steinway senior technician running his own operation doing a restoration.

@mijostyn As a New England neighbor of mine, when appropriately safe, you (& significant other/family) are more than welcome head south of the border to MA and listen to the different setups I’m still trying to make sense out of. Your experience/inputs of course, would be of great assistance and much appreciated.

@hleeid- on the question of which humidification systems, I cannot tell you- it has been too long and several lifetimes ago. I'd just talk to a specialist in vintage pianos and ask them. How old is yours?