NH-D15 Versus 240mm AIO - Cooling - Linus Tech Tips

I'm having some perplexing problems with the NH-D15.  All sources I can find have declared that this air cooler performs as good or better than pretty much any AIO on the market -- even the ones that outperform it do so by small margins.  

 

I'm having the opposite experience.  Compared to an inexpensive Cooler Master 240mm AIO, the air cooler is consistently 5-10 degrees warmer regardless of configuration.  This isn't necessarily a problem (the air cooler typically runs between 70-75 versus the AIO rarely getting above 70), but given the fairly consistent conclusions across sources that the NH-D15 is indistinguishable from AIO's for performance, I can only conclude that I'm doing something wrong.   I'm using:

Fractal Design Meshify C

Ryzen 7 3700X

GeForce RTX 2080 Ti

2x 140mm fans

 

All fans are on their stock curves in all cases.  I've made it more aggressive and it helped a bit -- by lowering the temps of both coolers by similar amounts.  And making things loud.  So I undid it.

 

For the NH-D15 tests, I've put those 140mm fans everywhere the case will fit them -- front intake, top exhaust, top intake -- and 70-75c is the best it can do.  I don't use a rear fan since the NH-D15 has to be placed so close to the rear of the case that I suspect they'd just be interfering with each other (there are RAM clearance issues on the other side).  I even have the side of the case open with a box fan blowing room temp air into it and it isn't making a difference in temps (okay, maybe 1 degree or so).  But my suspicion that the GPU is raising ambient temps in the case is bunk -- the box fan would have improved things if that was the case, I'd think.   On the other hand, I can do whatever dopey configuration I want with the AIO and it outperforms the air cooler every time, and by notable margins in the best configurations.  I've even carelessly handled it so much that a lot of the fins are bent.  Still works better.

 

Are these results to be expected?  I'd like my system to run in the 60's under load if possible, but that AIO makes Frightening Water Noises™ and I don't want to put it in my new system.  Plus, there's the principle of the thing: I'm getting markedly different results from pretty much everybody on YouTube and I must know why.  

 

Thanks in advance for any thoughts you may have!