Just the other day a band came in for a quick tracking session. As they were setting up their gear I heard one of the guitar players comment that he wanted to buy a guitar preamp for $250 so he could run his guitar into it before hitting his cheap solid-state amp. I explained to him that a lot of the sound of a classic tube amp comes from the power tubes, and that he would still be running through the solid- state portion of the amp, therefore only marginally improving his sound. I don't think he really believed me.
The recording studio is the place where musical gear meets its moment of truth. It doesn't matter how much you spent on it, how long you've owned it, what color it is or what brand it is. If it doesn't sound good, it doesn't sound good.
One guy brought in a $100 bass and a guitar amp and said, "I want a really deep tone." I tried the bass through the guitar amp and determined there were very few low frequencies coming through the speakers so I decided to try just a DI. By the third DI box I decided it wasn't my gear's fault and that the cheap bass was also a crummy bass with very lousy pickups. All I could do was keep adding low end at every turn, which created a very muddy soup.
I once had a drummer come in who had, as the batter-side head of his kick drum, an old head from the front-side with a 6" hole in it. So he was beating on a drum head with a big fucking hole in it. He didn't seem to want to go buy a new one so I stuffed the drum with padding and contented myself with a muddy thump as the kick sound. When he came in for a session a year later the head was still on there but luckily Steve West (of Pavement) had left one laying around that didn't have a hole in it, so we used that instead.