A common piece of hardware for any musician is the amplifier head, but using a traditional head means packaging the preamp and power amp together, so your sections are set in advance. Choosing a standalone preamp is a good way around this limitation, giving you the opportunity to match up to the power amp of your choice. You'll find a number of rack-mounted and free-standing preamps in this lineup, which are the perfect partners for a high-end power amp in a personalized setup.
Of course, you're not limited to just one preamp, and many of the models here are designed to be used in addition to the one you've already got in your rack or studio equipment. Most of those come in the form of a pedal, giving you an extra layer of tone-shaping potential on top of what you get from your amp head or combo amplifier alone. Another option worth looking into for guitar and bass players is Aguilar's series of onboard preamps. They're compact preamps that fit right into the guitar body to beef up the sound signal before it even leaves the instrument.
No matter which type of preamp is right for you, there's no doubt that it will go a long way toward defining your individual sound and style. These amplifiers are a customizer's dream: considering that the preamp's role is to shape your tone from the ground up, there's simply no other piece of equipment with as much power to create exactly the sort of sound you want.