Double Bass: aim a cardioid microphone at one of the f holes from a distance of about 16 inches (40 cm). If you have to record the double bass playing together with an ensemble, use a shorter working distance and a hypercardioid pattern in order to prevent leakage from other instruments into the bass microphone.
Cello: mic the cello in the same way as the double bass using a cardioid and consider using an extra distant microphone. Set the level of the close-up microphone (the spot microphone in this setup) about 20 dB lower than the level of the distant (main) microphone.
