Something called Analog mastering is one of the best secrets of top studios now. As most everything is recorded in the digital realm, top mastering houses will do what's called layback mastering. The digital version is changed back into an analog version in order to be mastered through original analog processors, though a lot of times it is also recorded to an two-track system to acheive the fatness that original analog tape gives to the album. It also takes the brittleness away from digital recordings, and can round out the bottom end giving the bass frequencies that nice analog sound.
There are pros and cons but the advantages are worth it. Analog CD mastering will often cost a bit more than traditional all digital mastering, as it takes more time and there are other costs for the analog tape, but for serious projects, it can be the end step in achieving the top sound quality for the album. Since digital recordings sound harsh to many listeners because no matter how loud the sound gets, digital always gives you purely digital reproduction. Analog tape, however, saturates as the signal increases in level, so high-ends are smoothed over rather than clipped, which translates as a warm, fat sound as the harmonic series is smoothed over. To the audience, this is more pleasing. In addition, a hotter input sound will result in a bigger effect. As the highs soften up first, this means that as the signal increases, the highs will naturally smooth out a bit, creating a less cold, more pleasing sound. This is the beauty of the analog mastering method.
There are several mastering houses around the United States that have the tools to do analog mastering, however, it also necessitates you have an engineer with the proven experience to get the best end product possible with the tools. I have tested out a few studios and one of top industry is www.stevethomasmastering.com. He is a top analog CD mastering engineer, yet he also has done thousands of albums as a recording engineer as well. On top of that, his results rivals some of the most pricey mastering houses in the world. If you already have someone you are working with that's great. But, I would really encourage you to give him a call.