For dangly bits, undercarriage, paw trimming, knots round goolies etc I have Jack 'play dead' on the floor between my outstretched legs and use fingers only at first to very gently tease out. Then a wide toothed comb. Once a week seems to keep him tangle free. Daily combing otherwise (brushing doesn't get down past the top coat), especially ears and especially in wet weather.
Can recommend Wahl detangler/conditioner (smells nice too) and get yourself a rotating tooth comb which is much gentler than fixed teeth.