Possibly one of the biggest challenges to project studio owner/engineers is drum recording! This can be especially true if the engineer is not a drummer! You need to have good microphones, and a good-sounding room to start with. Of course, it also helps to have a good drummer!
Even the best microphones coupled with the best room sound won't help a poorly-tuned drum kit. Often, drummers who only play live gigs do not even think about tuning. In a live situation, especially for rock players, these subtleties are usually lost on the listener anyway. It's more important that the audience can hear the drums above the guitarist. Thus, some drummers never even learn to tune their kit, because they don't need to.
However, when you get into the studio, the sound of the drum kit is suddenly under a microscope, and tuning becomes essential! Old drum heads, old, cracked cymbals, and other hardware defects can also adversely affect the sound. As an engineer, there are several things you can do:
1. Learn how to tune a drum kit yourself. This includes having the hardware on hand! You may need to keep several drum heads, as well as a few cymbals, around as well.
2. Buy a fairly decent drum kit for your studio, keep it well in tune, and insist that all drummers use your kit. If you have the money, this is a great option. You can even keep them set up and miked, which will save you loads of time. If you can't afford an entire drum kit, you might even think about buying a high-quality snare. Believe me, this is worth it. If you consistently have great-sounding drum tracks, you may get quite a bit of work because of it!
I might also add that it is equally possible that a guitarist might walk into your studio with a terrible-sounding rig, so it wouldn't hurt to purchase at least one decent guitar amp as well. The bottom line: A good studio often requires more than just the recording equipment.