Most older (vintage) preamps do not have a detachable power cord.

What is your budget?

Do you want/need tone controls?
If you do, then something like an Adcom GFP-555 is an ideal choice. It also fits your cosmetic requirements.

Most of the preamps I would recommend have no tone controls.

The ones that come to mind are

Audio Research
Conrad Johnson
Krell
Audible Illusions

All of the above are considered to be high end preamps. They can be had for less than a grand.

Check Audiogon for used preamps. If you are on a restricted budget the already mentioned Adcom is probably your best bet.