put tube amp on to shelf of a rack

First, "AC Infinity", makes a fully programable rack type fan system that you can get with the venting either in the front, top or rear of the units, I have several and have used them at times when testing new transducers I've built with loads only calculated theoretically before testing.
These can be mounted "Under" the metal frames in your rack at only 1.5" thick and move plenty of air. "It is allot of fan and tech. for the price also".. They use a infrared, "Probe" to sense the gears heat and then cool it to your preference OR send an alarm If it cannot keep up. And if it cannot? You definitely have a problem! They saved several amps from melting at a certain point! "Those darn and, (O-So -Pesky), 0 Ohm loads! They are just trying to "double down" into infinity!
https://www.acinfinity.com/component-cooling/component-fan-systems/aircom-t8-receiver-and-av-component-cooling-blower-system-rear-exhaust-17/#product-warranty
I liked their stuff so much I bought a bunch of the different types/models of the fans that they make. Look around the site and you'll see. 
They are very quiet also. Even when the three fan T-8 is all the way up in fan speed it is still much quieter then my Krell, monoblocks!
The Racks you have can be problematic with that much weight. "I own a bunch of those". But I did have a Pass Lab's, X-350 sitting in the top middle position in one rig and never had a problem!
"I believe that amp is 120-125Lbs.". If you watch "You-tube" there are videos from McIntosh showing the unboxing and installation of their larger amps with which they recommend using, "4" , people to lift and then place into home systems. I'm a big guy, but I will admit that If I had tried to first lift from the floor, twist or walk a few steps into a position to then, "Gingerly, lean forward and place a huge amplifier into an fairly exact position"?
 I would probably have destroyed part of my back. As it is, even with two strong people? Certain heavy gear is still a trial simply because much of it is so large and cumbersome, as well as being heavy! But the fella I sold a McIntosh MC-402, that I sold "Insisted" that he could place it into his SUV by himself. He regretted that almost immediately after doing so.

@jollygreenaudiophile2 Hey thanks for the detailed information. I ordered two 120mm AC Infinity usb powered fans as they can be blutacked on the floor where my VAC tube power amp sits on right now. 

And I can't agree more about the lifting heavy amp part. As age catches up I no longer dare take the risk lifting equips around 100lb by myself. Not to say slide them into tight spaces. If I have to, I need to get the right set of tools that make me comfortable doing so.

"Hwa Rang Do"? "Carson city, Century city or Seoul"?
In any case, Your quite welcome, "Yuhengdu T."!
I will add though; They do offer "separately", A very nice filtering system you may add to any of the fans that they make/sell. It is a simple "Injection molded", filter housing which comes with a, "washable and re-usable", foam filter. You attach them as you will. "They were not any help at all when I called them about this".
But the filter housings have a hole-thru, in each corner. "I simply used self-tapping, (Tappets)". And I installed three each on my "T-8" units. Two each on my "S2's", (And so-on). And I have maybe a case of them left if you would like a few. "They described them over the phone where I ordered them not understanding that they were re-usable. And they were very hard to find on the web-site for some reason....."I never have seen a pic of these"!
 Adding the filters to the system made a lot of sense to me and gives you that, "Warm and fuzzy", that is o-so-satisfying once you add them!

The link shown in your post is not enough space between shelves for all those tubes, IMHO. Even with fans I wouldn’t do it. Get yourself a separate amp stand if you want the amp down low, but heat rises, so I’d personally have the amp more at eye level. That’s an amp that deserves to be on the top shelf, I mean who doesn’t like to look at the glow of tubes. If you’ve got it, flaunt it!