Take the feet off and rack it. That's what I did; never gave me a problem and the rack I had did not have much vent space. It still never overheated ever. The feet are there for use when your not racking; mostly so the amp will not slide around on your cabinet.
"But lay up for yourselves treasures in heaven, where neither moth nor rust doth corrupt, and where thieves do not break through nor steal:" Matthew 6:20