I wouldn't know Philtone if it slapped me up'side the head (sorry)... But Charles has written a very nice piece here, about how to approach SF-2 settings, so I'll try to help out a little.
I especially like the concept of listening to just one filter, until you find some aspect that you like, and then working on blending it back in with the rest of the sound. This works well for me, and Charles has provided a great example.
As for the gradual effect of using the bandpass vs. the lowpass in the low end of the neighborhood...
On Channel A, Charles has the frequency set all the way down to 45, with Q/Damping at a moderate 3. So there are two things going on here.
First, the Q is giving him modest boost in a range somewhere around the low E or F. I don't know exactly what the range is, but it probably is making a difference down to B or C, certainly D, as well as somewhat higher (probably up to A, maybe B).
Second, the low pass filter uses a 12 dB/octave slope, while the band pass uses a 6 dB slope. So once you get above the boost region, the band pass setting is progressively reducing the level, by half as much per octave compared to using a low pass filter. That's what he means by gradual.
Charles is getting a pretty good boost on the lowest few notes of his 5 string, and gradually ramping down (as he plays higher) from there. Assuming you (Bill) were using the same Damping, but a low pass at a higher frequency, then you would be getting a much sharper peak around whatever frequency you had set.
Even if the Damping (Q) was the same, you would not be boosting the lowest notes as much, and above the frequency setting, you would be cutting them off twice as fast. In other words, Charles is getting a nice gradual increase as he goes down the scale, with some extra boost near the very bottom. In contrast, you are getting a sharper and narrower peak somewhere above the bottom.
It's a little hard to explain, especially without drawing some pictures. I encourage you to try Charles' approach: turn off the direct gain and the other filter gain, and try both settings (better yet, set one filter each way, and alternately use one or the other).
Hope that helps, a little.
-Bob