The HB sized P-90 is loud! It is louder than my mini-Mule, even though the mini-Mule is in the neck position and the P-90 in the bridge. However the tone is briliant so I am quite happy to live with the extra volume! Perhaps I should have gone for alnico V in the min-Mule (I think mine has IV's), but I never expected so much sound from an 8K P-90.
It is also pretty quiet until the gain is turned up. I guess the metal cover makes a kind of Faraday box for the pickup and helps cut down the hum.
The coil tap changes the voicing more than the volume. It gets less mids and a brighter tone on the tap.