With an open-back amp, getting a closed-back sound might be easier than you think
Open-back amps have less bass response than closed-back amps, but both types have their uses, particularly in the studio. It’s not too hard to emulate an open-back sound with a recorded closed-back ampd – just use some EQ to thin out the bass end a bit. It’s a little harder to do the reverse, because a closed-back amp sort of “compresses” the speaker so the sound is tighter, not just bassier.
So referring to the image above, there’s a simple solution: place the open back amp so the back faces down on a rug, and point a mic down at it. However, there is an important caution. If the amp has tubes in it, to avoid heat buildup, you don’t want to block the ventilation for any more time than is absolutely necessary.