coyote

First my question:  How do I (temporarily) remove the Core V71's front (bottom) feet?
I ask in order to install the bottom (120mm) fans, which has been a frustrating project.
The first things I tried (in part to avoid having to deal with removing the feet) were rubber fan mounts.
First, I  tried
https://amazon.com/gp/product/B0749BRLXW/

but their base turned out to be too thick to not warp the air filter.  So next I tried
https://amazon.com/gp/product/B003JBOBOI/
(because it's got a very thin plastic screw that can be outside the case all by itself so it won't warp the air filter)
but as you can see

that's going to leave big gap (5 mm!) all around the fan (so the fan wouldn't bother to pull as much air through the filter).  But other than that (lol) they worked.
So I decided to use standard case fan screws
https://amazon.com/GCA-Computer-Cooling-Mount-Screws/dp/B083NTT1KN/
which look like this:

(and the case does come with enough--four--to mount one of the two fans)
However, being screws, one needs to be able to use a screwdriver on them, and under the case (for the front-most row of holes) in the way of one's screwdriver are the case's plastic front feet, as can be seen here:

And the only attachments that seem owner-adjustable are these plastic clips:

(there's another one that looks like this, and I would need to remove the HDD cage mounts to get at it)
I've tried pushing this plastic clip with a great deal of force, but it hasn't budged the slightest bit.  
I'm rather angry with the Core 71 now.  These bottom fan positions were a deciding factor in my purchase.
On a positive note, the big ventilation holes are just the right size for PC case screws (though a bit too small for the first rubber fan mounts I tried).

I'd like the share my answer to my (and others') question of whether a Noctua 200mm fan will fit in the TT Core V71.  (Ditto a 200mm BitFenix BFF-LPRO-23030B-RP I already had.)
Note than I only looked at this question WRT the front fans (sorry, I plan to put a radiator in the top).
For neither alternate 200mm fan is thickness an issue; while I'm not sure you can tell from the photos, I think both are exactly the same thickness as the stock TT 200mm fans.  This is important because the mesh filter is right up against the fans.
And mounting holes are not an issue for either alternative fan either; one of the sets of holes for each alternative fan matched the case holes.
However, the stock TT fan width is perfectly minimal to fit the case, and both alternative fans are wider, and would thus need to be modified to fit.
The Noctua fan's inner edge hole is all one would need; to fit inside the front panel attachments the rest of the four edge extensions would need to be cut off.  The plastic is hard and I think resilient.  
The BitFenix fan is nearly circled by holes; nearly all of them would need to be cut off the fan.  The plastic is hard but also I think brittle.
To complicate matters further, there's the manual warning about using other fans burning out the controller.  (Furthermore, when I owned a similar TT case a dozen years ago with only one front 200mm fan, I vaguely recall something I did wrong connecting it burned out the fan; I think I may have figured that out and got an alternate  replacement working.)
With all that, I don't think I'm going to deal with modifying alternative 200mm fans to fit the case in the short term.
The pictured Noctua is a Noctua NF-A20 PWM chromax.black.swap

I'd like the share my answer to my (and others') question of whether a Noctua 200mm fan will fit in the TT Core V71.  (Ditto a 200mm BitFenix BFF-LPRO-23030B-RP I already had.)
Note than I only looked at this question WRT the front fans (sorry, I plan to put a radiator in the top).
For neither alternate 200mm fan is thickness an issue; while I'm not sure you can tell from the photos, I think both are exactly the same thickness as the stock TT 200mm fans.  This is important because the mesh filter is right up against the fans.
And mounting holes are not an issue for either alternative fan either; one of the sets of holes for each alternative fan matched the case holes.
However, the stock TT fan width is perfectly minimal to fit the case, and both alternative fans are wider, and would thus need to be modified to fit.
The Noctua fan's inner edge hole is all one would need; to fit inside the front panel attachments the rest of the four edge extensions would need to be cut off.  The plastic is hard and I think resilient.  
The BitFenix fan is nearly circled by holes; nearly all of them would need to be cut off the fan.  The plastic is hard but also I think brittle.
To complicate matters further, there's the manual warning about using other fans burning out the controller.  (Furthermore, when I owned a similar TT case a dozen years ago with only one front 200mm fan, I vaguely recall something I did wrong connecting it burned out the fan; I think I may have figured that out and got an alternate  replacement working.)
With all that, I don't think I'm going to deal with modifying alternative 200mm fans to fit the case in the short term.
The pictured Noctua is a Noctua NF-A20 PWM chromax.black.swap

Hmm, ya, I see - why not just use small cable ties to fix the fans and you only need 2 ties (diagonal mount) for each fan.