I've written about the B&W headphones several times in the past. First I complained about the fit of the C7. Then I discovered that by using the largest eartips I could get an excellent fit with the C7s, especially after the tips had fully expanded.
Some P5 owners have taken to modifying B&W's Flagship P5s to increase their bass response. The mod seems to be quite simple; it involves cutting back the ear cushion memory foam so that it doesn't block the edges of the P5's driver's diaphragm. And by removing some of the foam the end result is larger air chamber in front of the diaphragm, which modifiers claim gives "more" bass.
Personally, I don't feel the need for more bass from my P3 headphones, which use the same ear-foam design, so I haven't done the modification. But if you do feel the need for additional whomp, you could try it. Fortunately it only alters the replaceable foam ear cushions, so it is reversible (as long as you purchase a second set of cushions). A replacement set runs around $30 in the U.S.
While I'm not a huge fan of irreversible modifications to components, I see no reason why not to use reversable mods, such as this one. It is roughly akin to changing ear tips on in-ear monitors to alter the fit, which will affect the harmonic balance.