What advice would you give a customer looking to hire a provider in your area of work?
Make sure that in addition to hearing samples of the completed work done in the studio also go and listen to what the studio sounds like before the engineer works their magic on it. After all the ROOM is what effects the sound more than anything else and a bad sounding room can not be fixed in the mix.
Also ask the studio what mics and mic pres they have available for you to use. Microphones are unique and should be selected for each performer based upon the instrument or vocalists sound. A studio with only one mic and one mic pre recording in a closet will always have the same sound for each artist.