For a while now I have been running an ampeg svt DI between my bass' and my ss amp. Although it was sold to me on the basis that it would fatten my tone up (tube buffered DI) I have found that it really only stops the amp from clipping so much so I get a better sound anyway. However I just plugged the T40 into it and wham! a totally fat tone, really noticable... so I figured maybe I didnt get it. Plugged the fender p and no discernable sound difference again. So I plugged in my yamaha with similar humbucking pickups like the T40 and I got the fat tone again! Anyone got an explanation or am I going insane???

Humbuckers put out more volume then singles, so they push the tubes harder. Put a booster after your p=bass for a like effect (to a point, if you put out too much volume you'll get some distortion, which may or may not be what your going for).

Yeah, I was starting to think that too. I am going to get a compressor stomp box to do that so I can have the same effect for the p bass. thanks!